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| Meta Title | Calculus III - Higher Order Partial Derivatives |
| Meta Description | In the section we will take a look at higher order partial derivatives. Unlike Calculus I however, we will have multiple second order derivatives, multiple third order derivatives, etc. because we are now working with functions of multiple variables. We will also discuss Clairaut’s Theorem to help with some of the work in finding higher order derivatives. |
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Section 13.4 : Higher Order Partial Derivatives
Just as we had higher order derivatives with functions of one variable we will also have higher order derivatives of functions of more than one variable. However, this time we will have more options since we do have more than one variable.
Consider the case of a function of two variables, \(f\left( {x,y} \right)\) since both of the first order partial derivatives are also functions of \(x\) and \(y\) we could in turn differentiate each with respect to \(x\) or \(y\). This means that for the case of a function of two variables there will be a total of four possible second order derivatives. Here they are and the notations that we’ll use to denote them.
\[\begin{align*}{\left( {{f_x}} \right)_x} & = {f_{x\,x}} = \frac{\partial }{{\partial x}}\left( {\frac{{\partial f}}{{\partial x}}} \right) = \frac{{{\partial ^2}f}}{{\partial {x^2}}}\\ {\left( {{f_x}} \right)_y} & = {f_{x\,y}} = \frac{\partial }{{\partial y}}\left( {\frac{{\partial f}}{{\partial x}}} \right) = \frac{{{\partial ^2}f}}{{\partial y\partial x}}\\ {\left( {{f_y}} \right)_x} & = {f_{y\,x}} = \frac{\partial }{{\partial x}}\left( {\frac{{\partial f}}{{\partial y}}} \right) = \frac{{{\partial ^2}f}}{{\partial x\partial y}}\\ {\left( {{f_y}} \right)_y} & = {f_{y\,y}} = \frac{\partial }{{\partial y}}\left( {\frac{{\partial f}}{{\partial y}}} \right) = \frac{{{\partial ^2}f}}{{\partial {y^2}}}\end{align*}\]
The second and third second order partial derivatives are often called mixed partial derivatives since we are taking derivatives with respect to more than one variable. Note as well that the order that we take the derivatives in is given by the notation for each these. If we are using the subscripting notation,
e.g.
\({f_{x\,y}}\), then we will differentiate from left to right. In other words, in this case, we will differentiate first with respect to \(x\) and then with respect to \(y\). With the fractional notation,
e.g.
\(\frac{{{\partial ^2}f}}{{\partial y\partial x}}\), it is the opposite. In these cases we differentiate moving along the denominator from right to left. So, again, in this case we differentiate with respect to \(x\) first and then \(y\).
Let’s take a quick look at an example.
Example 1
Find all the second order derivatives for \(f\left( {x,y} \right) = \cos \left( {2x} \right) - {x^2}{{\bf{e}}^{5y}} + 3{y^2}\).
Show Solution
We’ll first need the first order derivatives so here they are.
\[\begin{align*}{f_x}\left( {x,y} \right) & = - 2\sin \left( {2x} \right) - 2x{{\bf{e}}^{5y}}\\ {f_y}\left( {x,y} \right) & = - 5{x^2}{{\bf{e}}^{5y}} + 6y\end{align*}\]
Now, let’s get the second order derivatives.
\[\begin{align*}{f_{xx}} & = - 4\cos \left( {2x} \right) - 2{{\bf{e}}^{5y}}\\ {f_{xy}} & = - 10x{{\bf{e}}^{5y}}\\ {f_{yx}} & = - 10x{{\bf{e}}^{5y}}\\ {f_{yy}} & = - 25{x^2}{{\bf{e}}^{5y}} + 6\end{align*}\]
Notice that we dropped the \(\left( {x,y} \right)\) from the derivatives. This is fairly standard and we will be doing it most of the time from this point on. We will also be dropping it for the first order derivatives in most cases.
Now let’s also notice that, in this case, \({f_{xy}} = {f_{yx}}\). This is not by coincidence. If the function is “nice enough” this will always be the case. So, what’s “nice enough”? The following theorem tells us.
Clairaut’s Theorem
Suppose that \(f\) is defined on a disk \(D\) that contains the point \(\left( {a,b} \right)\). If the functions \({f_{xy}}\) and \({f_{yx}}\) are continuous on this disk then,
\[{f_{xy}}\left( {a,b} \right) = {f_{yx}}\left( {a,b} \right)\]
Now, do not get too excited about the disk business and the fact that we gave the theorem for a specific point. In pretty much every example in this class if the two mixed second order partial derivatives are continuous then they will be equal.
Example 2
Verify Clairaut’s Theorem for \(f\left( {x,y} \right) = x{{\bf{e}}^{ - {x^2}{y^2}}}\).
Show Solution
We’ll first need the two first order derivatives.
\[\begin{align*}{f_x}\left( {x,y} \right) & = {{\bf{e}}^{ - {x^2}{y^2}}} - 2{x^2}{y^2}{{\bf{e}}^{ - {x^2}{y^2}}}\\ {f_y}\left( {x,y} \right) & = - 2y{x^3}{{\bf{e}}^{ - {x^2}{y^2}}}\end{align*}\]
Now, compute the two mixed second order partial derivatives.
\[\begin{align*}{f_{xy}}\left( {x,y} \right) & = - 2y{x^2}{{\bf{e}}^{ - {x^2}{y^2}}} - 4{x^2}y{{\bf{e}}^{ - {x^2}{y^2}}} + 4{x^4}{y^3}{{\bf{e}}^{ - {x^2}{y^2}}} = - 6{x^2}y{{\bf{e}}^{ - {x^2}{y^2}}} + 4{x^4}{y^3}{{\bf{e}}^{ - {x^2}{y^2}}}\\ {f_{yx}}\left( {x,y} \right) & = - 6y{x^2}{{\bf{e}}^{ - {x^2}{y^2}}} + 4{y^3}{x^4}{{\bf{e}}^{ - {x^2}{y^2}}}\end{align*}\]
Sure enough they are the same.
So far we have only looked at second order derivatives. There are, of course, higher order derivatives as well. Here are a couple of the third order partial derivatives of function of two variables.
\[\begin{align*}{f_{x\,y\,x}} & = {\left( {{f_{xy}}} \right)_x} = \frac{\partial }{{\partial x}}\left( {\frac{{{\partial ^2}f}}{{\partial y\partial x}}} \right) = \frac{{{\partial ^3}f}}{{\partial x\partial y\partial x}}\\ {f_{y\,x\,x}} & = {\left( {{f_{yx}}} \right)_x} = \frac{\partial }{{\partial x}}\left( {\frac{{{\partial ^2}f}}{{\partial x\partial y}}} \right) = \frac{{{\partial ^3}f}}{{\partial {x^2}\partial y}}\end{align*}\]
Notice as well that for both of these we differentiate once with respect to \(y\) and twice with respect to \(x\). There is also another third order partial derivative in which we can do this, \({f_{x\,x\,y}}\). There is an extension to Clairaut’s Theorem that says if all three of these are continuous then they should all be equal,
\[{f_{x\,x\,y}} = {f_{x\,y\,x}} = {f_{y\,x\,x}}\]
To this point we’ve only looked at functions of two variables, but everything that we’ve done to this point will work regardless of the number of variables that we’ve got in the function and there are natural extensions to Clairaut’s theorem to all of these cases as well. For instance,
\[{f_{x\,z}}\left( {x,y,z} \right) = {f_{z\,x}}\left( {x,y,z} \right)\]
provided both of the derivatives are continuous.
In general, we can extend Clairaut’s theorem to any function and mixed partial derivatives. The only requirement is that in each derivative we differentiate with respect to each variable the same number of times. In other words, provided we meet the continuity condition, the following will be equal
\[{f_{s\,s\,r\,t\,s\,r\,r}} = {f_{t\,r\,s\,r\,s\,s\,r}}\]
because in each case we differentiate with respect to \(t\) once, \(s\) three times and \(r\) three times.
Let’s do a couple of examples with higher (well higher order than two anyway) order derivatives and functions of more than two variables.
Example 3
Find the indicated derivative for each of the following functions.
Find \({f_{x\,x\,y\,z\,z}}\) for \(f\left( {x,y,z} \right) = {z^3}{y^2}\ln \left( x \right)\)
Find \(\displaystyle \frac{{{\partial ^3}f}}{{\partial y\partial {x^2}}}\) for \(f\left( {x,y} \right) = {{\bf{e}}^{xy}}\)
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Â
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a
Find \({f_{x\,x\,y\,z\,z}}\) for \(f\left( {x,y,z} \right) = {z^3}{y^2}\ln \left( x \right)\)
Show Solution
In this case remember that we differentiate from left to right. Here are the derivatives for this part.
\[{f_x} = \frac{{{z^3}{y^2}}}{x}\]
\[{f_{xx}} = - \frac{{{z^3}{y^2}}}{{{x^2}}}\]
\[{f_{xxy}} = - \frac{{2{z^3}y}}{{{x^2}}}\]
\[{f_{xxyz}} = - \frac{{6{z^2}y}}{{{x^2}}}\]
\[{f_{xxyzz}} = - \frac{{12zy}}{{{x^2}}}\]
b
Find \(\displaystyle \frac{{{\partial ^3}f}}{{\partial y\partial {x^2}}}\) for \(f\left( {x,y} \right) = {{\bf{e}}^{xy}}\)
Show Solution
Here we differentiate from right to left. Here are the derivatives for this function.
\[\frac{{\partial f}}{{\partial x}} = y{{\bf{e}}^{xy}}\]
\[\frac{{{\partial ^2}f}}{{\partial {x^2}}} = {y^2}{{\bf{e}}^{xy}}\]
\[\frac{{{\partial ^3}f}}{{\partial y\partial {x^2}}} = 2y{{\bf{e}}^{xy}} + x{y^2}{{\bf{e}}^{xy}}\] |
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- [4\.8 Optimization](https://tutorial.math.lamar.edu/Classes/CalcI/Optimization.aspx)
- [4\.9 More Optimization Problems](https://tutorial.math.lamar.edu/Classes/CalcI/MoreOptimization.aspx)
- [4\.10 L'Hospital's Rule and Indeterminate Forms](https://tutorial.math.lamar.edu/Classes/CalcI/LHospitalsRule.aspx)
- [4\.11 Linear Approximations](https://tutorial.math.lamar.edu/Classes/CalcI/LinearApproximations.aspx)
- [4\.12 Differentials](https://tutorial.math.lamar.edu/Classes/CalcI/Differentials.aspx)
- [4\.13 Newton's Method](https://tutorial.math.lamar.edu/Classes/CalcI/NewtonsMethod.aspx)
- [4\.14 Business Applications](https://tutorial.math.lamar.edu/Classes/CalcI/BusinessApps.aspx)
- [5\. Integrals](https://tutorial.math.lamar.edu/Classes/CalcI/IntegralsIntro.aspx)
- [5\.1 Indefinite Integrals](https://tutorial.math.lamar.edu/Classes/CalcI/IndefiniteIntegrals.aspx)
- [5\.2 Computing Indefinite Integrals](https://tutorial.math.lamar.edu/Classes/CalcI/ComputingIndefiniteIntegrals.aspx)
- [5\.3 Substitution Rule for Indefinite Integrals](https://tutorial.math.lamar.edu/Classes/CalcI/SubstitutionRuleIndefinite.aspx)
- [5\.4 More Substitution Rule](https://tutorial.math.lamar.edu/Classes/CalcI/SubstitutionRuleIndefinitePtII.aspx)
- [5\.5 Area Problem](https://tutorial.math.lamar.edu/Classes/CalcI/AreaProblem.aspx)
- [5\.6 Definition of the Definite Integral](https://tutorial.math.lamar.edu/Classes/CalcI/DefnOfDefiniteIntegral.aspx)
- [5\.7 Computing Definite Integrals](https://tutorial.math.lamar.edu/Classes/CalcI/ComputingDefiniteIntegrals.aspx)
- [5\.8 Substitution Rule for Definite Integrals](https://tutorial.math.lamar.edu/Classes/CalcI/SubstitutionRuleDefinite.aspx)
- [6\. Applications of Integrals](https://tutorial.math.lamar.edu/Classes/CalcI/IntAppsIntro.aspx)
- [6\.1 Average Function Value](https://tutorial.math.lamar.edu/Classes/CalcI/AvgFcnValue.aspx)
- [6\.2 Area Between Curves](https://tutorial.math.lamar.edu/Classes/CalcI/AreaBetweenCurves.aspx)
- [6\.3 Volumes of Solids of Revolution / Method of Rings](https://tutorial.math.lamar.edu/Classes/CalcI/VolumeWithRings.aspx)
- [6\.4 Volumes of Solids of Revolution/Method of Cylinders](https://tutorial.math.lamar.edu/Classes/CalcI/VolumeWithCylinder.aspx)
- [6\.5 More Volume Problems](https://tutorial.math.lamar.edu/Classes/CalcI/MoreVolume.aspx)
- [6\.6 Work](https://tutorial.math.lamar.edu/Classes/CalcI/Work.aspx)
- [Appendix A. Extras](https://tutorial.math.lamar.edu/Classes/CalcI/ExtrasIntro.aspx)
- [A.1 Proof of Various Limit Properties](https://tutorial.math.lamar.edu/Classes/CalcI/LimitProofs.aspx)
- [A.2 Proof of Various Derivative Properties](https://tutorial.math.lamar.edu/Classes/CalcI/DerivativeProofs.aspx)
- [A.3 Proof of Trig Limits](https://tutorial.math.lamar.edu/Classes/CalcI/ProofTrigDeriv.aspx)
- [A.4 Proofs of Derivative Applications Facts](https://tutorial.math.lamar.edu/Classes/CalcI/DerivativeAppsProofs.aspx)
- [A.5 Proof of Various Integral Properties](https://tutorial.math.lamar.edu/Classes/CalcI/ProofIntProp.aspx)
- [A.6 Area and Volume Formulas](https://tutorial.math.lamar.edu/Classes/CalcI/Area_Volume_Formulas.aspx)
- [A.7 Types of Infinity](https://tutorial.math.lamar.edu/Classes/CalcI/TypesOfInfinity.aspx)
- [A.8 Summation Notation](https://tutorial.math.lamar.edu/Classes/CalcI/SummationNotation.aspx)
- [A.9 Constant of Integration](https://tutorial.math.lamar.edu/Classes/CalcI/ConstantofIntegration.aspx)
- [Calculus II](https://tutorial.math.lamar.edu/Classes/CalcII/CalcII.aspx)
- [7\. Integration Techniques](https://tutorial.math.lamar.edu/Classes/CalcII/IntTechIntro.aspx)
- [7\.1 Integration by Parts](https://tutorial.math.lamar.edu/Classes/CalcII/IntegrationByParts.aspx)
- [7\.2 Integrals Involving Trig Functions](https://tutorial.math.lamar.edu/Classes/CalcII/IntegralsWithTrig.aspx)
- [7\.3 Trig Substitutions](https://tutorial.math.lamar.edu/Classes/CalcII/TrigSubstitutions.aspx)
- [7\.4 Partial Fractions](https://tutorial.math.lamar.edu/Classes/CalcII/PartialFractions.aspx)
- [7\.5 Integrals Involving Roots](https://tutorial.math.lamar.edu/Classes/CalcII/IntegralsWithRoots.aspx)
- [7\.6 Integrals Involving Quadratics](https://tutorial.math.lamar.edu/Classes/CalcII/IntegralsWithQuadratics.aspx)
- [7\.7 Integration Strategy](https://tutorial.math.lamar.edu/Classes/CalcII/IntegrationStrategy.aspx)
- [7\.8 Improper Integrals](https://tutorial.math.lamar.edu/Classes/CalcII/ImproperIntegrals.aspx)
- [7\.9 Comparison Test for Improper Integrals](https://tutorial.math.lamar.edu/Classes/CalcII/ImproperIntegralsCompTest.aspx)
- [7\.10 Approximating Definite Integrals](https://tutorial.math.lamar.edu/Classes/CalcII/ApproximatingDefIntegrals.aspx)
- [8\. Applications of Integrals](https://tutorial.math.lamar.edu/Classes/CalcII/IntAppsIntro.aspx)
- [8\.1 Arc Length](https://tutorial.math.lamar.edu/Classes/CalcII/ArcLength.aspx)
- [8\.2 Surface Area](https://tutorial.math.lamar.edu/Classes/CalcII/SurfaceArea.aspx)
- [8\.3 Center of Mass](https://tutorial.math.lamar.edu/Classes/CalcII/CenterOfMass.aspx)
- [8\.4 Hydrostatic Pressure](https://tutorial.math.lamar.edu/Classes/CalcII/HydrostaticPressure.aspx)
- [8\.5 Probability](https://tutorial.math.lamar.edu/Classes/CalcII/Probability.aspx)
- [9\. Parametric Equations and Polar Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/ParametricIntro.aspx)
- [9\.1 Parametric Equations and Curves](https://tutorial.math.lamar.edu/Classes/CalcII/ParametricEqn.aspx)
- [9\.2 Tangents with Parametric Equations](https://tutorial.math.lamar.edu/Classes/CalcII/ParaTangent.aspx)
- [9\.3 Area with Parametric Equations](https://tutorial.math.lamar.edu/Classes/CalcII/ParaArea.aspx)
- [9\.4 Arc Length with Parametric Equations](https://tutorial.math.lamar.edu/Classes/CalcII/ParaArcLength.aspx)
- [9\.5 Surface Area with Parametric Equations](https://tutorial.math.lamar.edu/Classes/CalcII/ParaSurfaceArea.aspx)
- [9\.6 Polar Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/PolarCoordinates.aspx)
- [9\.7 Tangents with Polar Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/PolarTangents.aspx)
- [9\.8 Area with Polar Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/PolarArea.aspx)
- [9\.9 Arc Length with Polar Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/PolarArcLength.aspx)
- [9\.10 Surface Area with Polar Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/PolarSurfaceArea.aspx)
- [9\.11 Arc Length and Surface Area Revisited](https://tutorial.math.lamar.edu/Classes/CalcII/ArcLength_SurfaceArea.aspx)
- [10\. Series & Sequences](https://tutorial.math.lamar.edu/Classes/CalcII/SeriesIntro.aspx)
- [10\.1 Sequences](https://tutorial.math.lamar.edu/Classes/CalcII/Sequences.aspx)
- [10\.2 More on Sequences](https://tutorial.math.lamar.edu/Classes/CalcII/MoreSequences.aspx)
- [10\.3 Series - The Basics](https://tutorial.math.lamar.edu/Classes/CalcII/Series_Basics.aspx)
- [10\.4 Convergence/Divergence of Series](https://tutorial.math.lamar.edu/Classes/CalcII/ConvergenceOfSeries.aspx)
- [10\.5 Special Series](https://tutorial.math.lamar.edu/Classes/CalcII/Series_Special.aspx)
- [10\.6 Integral Test](https://tutorial.math.lamar.edu/Classes/CalcII/IntegralTest.aspx)
- [10\.7 Comparison Test/Limit Comparison Test](https://tutorial.math.lamar.edu/Classes/CalcII/SeriesCompTest.aspx)
- [10\.8 Alternating Series Test](https://tutorial.math.lamar.edu/Classes/CalcII/AlternatingSeries.aspx)
- [10\.9 Absolute Convergence](https://tutorial.math.lamar.edu/Classes/CalcII/AbsoluteConvergence.aspx)
- [10\.10 Ratio Test](https://tutorial.math.lamar.edu/Classes/CalcII/RatioTest.aspx)
- [10\.11 Root Test](https://tutorial.math.lamar.edu/Classes/CalcII/RootTest.aspx)
- [10\.12 Strategy for Series](https://tutorial.math.lamar.edu/Classes/CalcII/SeriesStrategy.aspx)
- [10\.13 Estimating the Value of a Series](https://tutorial.math.lamar.edu/Classes/CalcII/EstimatingSeries.aspx)
- [10\.14 Power Series](https://tutorial.math.lamar.edu/Classes/CalcII/PowerSeries.aspx)
- [10\.15 Power Series and Functions](https://tutorial.math.lamar.edu/Classes/CalcII/PowerSeriesandFunctions.aspx)
- [10\.16 Taylor Series](https://tutorial.math.lamar.edu/Classes/CalcII/TaylorSeries.aspx)
- [10\.17 Applications of Series](https://tutorial.math.lamar.edu/Classes/CalcII/TaylorSeriesApps.aspx)
- [10\.18 Binomial Series](https://tutorial.math.lamar.edu/Classes/CalcII/BinomialSeries.aspx)
- [11\. Vectors](https://tutorial.math.lamar.edu/Classes/CalcII/VectorsIntro.aspx)
- [11\.1 Vectors - The Basics](https://tutorial.math.lamar.edu/Classes/CalcII/Vectors_Basics.aspx)
- [11\.2 Vector Arithmetic](https://tutorial.math.lamar.edu/Classes/CalcII/VectorArithmetic.aspx)
- [11\.3 Dot Product](https://tutorial.math.lamar.edu/Classes/CalcII/DotProduct.aspx)
- [11\.4 Cross Product](https://tutorial.math.lamar.edu/Classes/CalcII/CrossProduct.aspx)
- [12\. 3-Dimensional Space](https://tutorial.math.lamar.edu/Classes/CalcII/3DSpace.aspx)
- [12\.1 The 3-D Coordinate System](https://tutorial.math.lamar.edu/Classes/CalcII/3DCoords.aspx)
- [12\.2 Equations of Lines](https://tutorial.math.lamar.edu/Classes/CalcII/EqnsOfLines.aspx)
- [12\.3 Equations of Planes](https://tutorial.math.lamar.edu/Classes/CalcII/EqnsOfPlanes.aspx)
- [12\.4 Quadric Surfaces](https://tutorial.math.lamar.edu/Classes/CalcII/QuadricSurfaces.aspx)
- [12\.5 Functions of Several Variables](https://tutorial.math.lamar.edu/Classes/CalcII/MultiVrbleFcns.aspx)
- [12\.6 Vector Functions](https://tutorial.math.lamar.edu/Classes/CalcII/VectorFunctions.aspx)
- [12\.7 Calculus with Vector Functions](https://tutorial.math.lamar.edu/Classes/CalcII/VectorFcnsCalculus.aspx)
- [12\.8 Tangent, Normal and Binormal Vectors](https://tutorial.math.lamar.edu/Classes/CalcII/TangentNormalVectors.aspx)
- [12\.9 Arc Length with Vector Functions](https://tutorial.math.lamar.edu/Classes/CalcII/VectorArcLength.aspx)
- [12\.10 Curvature](https://tutorial.math.lamar.edu/Classes/CalcII/Curvature.aspx)
- [12\.11 Velocity and Acceleration](https://tutorial.math.lamar.edu/Classes/CalcII/Velocity_Acceleration.aspx)
- [12\.12 Cylindrical Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/CylindricalCoords.aspx)
- [12\.13 Spherical Coordinates](https://tutorial.math.lamar.edu/Classes/CalcII/SphericalCoords.aspx)
- [Calculus III](https://tutorial.math.lamar.edu/Classes/CalcIII/CalcIII.aspx)
- [12\. 3-Dimensional Space](https://tutorial.math.lamar.edu/Classes/CalcIII/3DSpace.aspx)
- [12\.1 The 3-D Coordinate System](https://tutorial.math.lamar.edu/Classes/CalcIII/3DCoords.aspx)
- [12\.2 Equations of Lines](https://tutorial.math.lamar.edu/Classes/CalcIII/EqnsOfLines.aspx)
- [12\.3 Equations of Planes](https://tutorial.math.lamar.edu/Classes/CalcIII/EqnsOfPlanes.aspx)
- [12\.4 Quadric Surfaces](https://tutorial.math.lamar.edu/Classes/CalcIII/QuadricSurfaces.aspx)
- [12\.5 Functions of Several Variables](https://tutorial.math.lamar.edu/Classes/CalcIII/MultiVrbleFcns.aspx)
- [12\.6 Vector Functions](https://tutorial.math.lamar.edu/Classes/CalcIII/VectorFunctions.aspx)
- [12\.7 Calculus with Vector Functions](https://tutorial.math.lamar.edu/Classes/CalcIII/VectorFcnsCalculus.aspx)
- [12\.8 Tangent, Normal and Binormal Vectors](https://tutorial.math.lamar.edu/Classes/CalcIII/TangentNormalVectors.aspx)
- [12\.9 Arc Length with Vector Functions](https://tutorial.math.lamar.edu/Classes/CalcIII/VectorArcLength.aspx)
- [12\.10 Curvature](https://tutorial.math.lamar.edu/Classes/CalcIII/Curvature.aspx)
- [12\.11 Velocity and Acceleration](https://tutorial.math.lamar.edu/Classes/CalcIII/Velocity_Acceleration.aspx)
- [12\.12 Cylindrical Coordinates](https://tutorial.math.lamar.edu/Classes/CalcIII/CylindricalCoords.aspx)
- [12\.13 Spherical Coordinates](https://tutorial.math.lamar.edu/Classes/CalcIII/SphericalCoords.aspx)
- [13\. Partial Derivatives](https://tutorial.math.lamar.edu/Classes/CalcIII/PartialDerivsIntro.aspx)
- [13\.1 Limits](https://tutorial.math.lamar.edu/Classes/CalcIII/Limits.aspx)
- [13\.2 Partial Derivatives](https://tutorial.math.lamar.edu/Classes/CalcIII/PartialDerivatives.aspx)
- [13\.3 Interpretations of Partial Derivatives](https://tutorial.math.lamar.edu/Classes/CalcIII/PartialDerivInterp.aspx)
- [13\.4 Higher Order Partial Derivatives](https://tutorial.math.lamar.edu/Classes/CalcIII/HighOrderPartialDerivs.aspx)
- [13\.5 Differentials](https://tutorial.math.lamar.edu/Classes/CalcIII/Differentials.aspx)
- [13\.6 Chain Rule](https://tutorial.math.lamar.edu/Classes/CalcIII/ChainRule.aspx)
- [13\.7 Directional Derivatives](https://tutorial.math.lamar.edu/Classes/CalcIII/DirectionalDeriv.aspx)
- [14\. Applications of Partial Derivatives](https://tutorial.math.lamar.edu/Classes/CalcIII/PartialDerivAppsIntro.aspx)
- [14\.1 Tangent Planes and Linear Approximations](https://tutorial.math.lamar.edu/Classes/CalcIII/TangentPlanes.aspx)
- [14\.2 Gradient Vector, Tangent Planes and Normal Lines](https://tutorial.math.lamar.edu/Classes/CalcIII/GradientVectorTangentPlane.aspx)
- [14\.3 Relative Minimums and Maximums](https://tutorial.math.lamar.edu/Classes/CalcIII/RelativeExtrema.aspx)
- [14\.4 Absolute Minimums and Maximums](https://tutorial.math.lamar.edu/Classes/CalcIII/AbsoluteExtrema.aspx)
- [14\.5 Lagrange Multipliers](https://tutorial.math.lamar.edu/Classes/CalcIII/LagrangeMultipliers.aspx)
- [15\. Multiple Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/MultipleIntegralsIntro.aspx)
- [15\.1 Double Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/DoubleIntegrals.aspx)
- [15\.2 Iterated Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/IteratedIntegrals.aspx)
- [15\.3 Double Integrals over General Regions](https://tutorial.math.lamar.edu/Classes/CalcIII/DIGeneralRegion.aspx)
- [15\.4 Double Integrals in Polar Coordinates](https://tutorial.math.lamar.edu/Classes/CalcIII/DIPolarCoords.aspx)
- [15\.5 Triple Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/TripleIntegrals.aspx)
- [15\.6 Triple Integrals in Cylindrical Coordinates](https://tutorial.math.lamar.edu/Classes/CalcIII/TICylindricalCoords.aspx)
- [15\.7 Triple Integrals in Spherical Coordinates](https://tutorial.math.lamar.edu/Classes/CalcIII/TISphericalCoords.aspx)
- [15\.8 Change of Variables](https://tutorial.math.lamar.edu/Classes/CalcIII/ChangeOfVariables.aspx)
- [15\.9 Surface Area](https://tutorial.math.lamar.edu/Classes/CalcIII/SurfaceArea.aspx)
- [15\.10 Area and Volume Revisited](https://tutorial.math.lamar.edu/Classes/CalcIII/Area_Volume.aspx)
- [16\. Line Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/LineIntegralsIntro.aspx)
- [16\.1 Vector Fields](https://tutorial.math.lamar.edu/Classes/CalcIII/VectorFields.aspx)
- [16\.2 Line Integrals - Part I](https://tutorial.math.lamar.edu/Classes/CalcIII/LineIntegralsPtI.aspx)
- [16\.3 Line Integrals - Part II](https://tutorial.math.lamar.edu/Classes/CalcIII/LineIntegralsPtII.aspx)
- [16\.4 Line Integrals of Vector Fields](https://tutorial.math.lamar.edu/Classes/CalcIII/LineIntegralsVectorFields.aspx)
- [16\.5 Fundamental Theorem for Line Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/FundThmLineIntegrals.aspx)
- [16\.6 Conservative Vector Fields](https://tutorial.math.lamar.edu/Classes/CalcIII/ConservativeVectorField.aspx)
- [16\.7 Green's Theorem](https://tutorial.math.lamar.edu/Classes/CalcIII/GreensTheorem.aspx)
- [17\.Surface Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/SurfaceIntegralsIntro.aspx)
- [17\.1 Curl and Divergence](https://tutorial.math.lamar.edu/Classes/CalcIII/CurlDivergence.aspx)
- [17\.2 Parametric Surfaces](https://tutorial.math.lamar.edu/Classes/CalcIII/ParametricSurfaces.aspx)
- [17\.3 Surface Integrals](https://tutorial.math.lamar.edu/Classes/CalcIII/SurfaceIntegrals.aspx)
- [17\.4 Surface Integrals of Vector Fields](https://tutorial.math.lamar.edu/Classes/CalcIII/SurfIntVectorField.aspx)
- [17\.5 Stokes' Theorem](https://tutorial.math.lamar.edu/Classes/CalcIII/StokesTheorem.aspx)
- [17\.6 Divergence Theorem](https://tutorial.math.lamar.edu/Classes/CalcIII/DivergenceTheorem.aspx)
- [Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/DE.aspx)
- [1\. Basic Concepts](https://tutorial.math.lamar.edu/Classes/DE/IntroBasic.aspx)
- [1\.1 Definitions](https://tutorial.math.lamar.edu/Classes/DE/Definitions.aspx)
- [1\.2 Direction Fields](https://tutorial.math.lamar.edu/Classes/DE/DirectionFields.aspx)
- [1\.3 Final Thoughts](https://tutorial.math.lamar.edu/Classes/DE/FinalThoughts.aspx)
- [2\. First Order DE's](https://tutorial.math.lamar.edu/Classes/DE/IntroFirstOrder.aspx)
- [2\.1 Linear Equations](https://tutorial.math.lamar.edu/Classes/DE/Linear.aspx)
- [2\.2 Separable Equations](https://tutorial.math.lamar.edu/Classes/DE/Separable.aspx)
- [2\.3 Exact Equations](https://tutorial.math.lamar.edu/Classes/DE/Exact.aspx)
- [2\.4 Bernoulli Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/Bernoulli.aspx)
- [2\.5 Substitutions](https://tutorial.math.lamar.edu/Classes/DE/Substitutions.aspx)
- [2\.6 Intervals of Validity](https://tutorial.math.lamar.edu/Classes/DE/IoV.aspx)
- [2\.7 Modeling with First Order DE's](https://tutorial.math.lamar.edu/Classes/DE/Modeling.aspx)
- [2\.8 Equilibrium Solutions](https://tutorial.math.lamar.edu/Classes/DE/EquilibriumSolutions.aspx)
- [2\.9 Euler's Method](https://tutorial.math.lamar.edu/Classes/DE/EulersMethod.aspx)
- [3\. Second Order DE's](https://tutorial.math.lamar.edu/Classes/DE/IntroSecondOrder.aspx)
- [3\.1 Basic Concepts](https://tutorial.math.lamar.edu/Classes/DE/SecondOrderConcepts.aspx)
- [3\.2 Real & Distinct Roots](https://tutorial.math.lamar.edu/Classes/DE/RealRoots.aspx)
- [3\.3 Complex Roots](https://tutorial.math.lamar.edu/Classes/DE/ComplexRoots.aspx)
- [3\.4 Repeated Roots](https://tutorial.math.lamar.edu/Classes/DE/RepeatedRoots.aspx)
- [3\.5 Reduction of Order](https://tutorial.math.lamar.edu/Classes/DE/ReductionofOrder.aspx)
- [3\.6 Fundamental Sets of Solutions](https://tutorial.math.lamar.edu/Classes/DE/FundamentalSetsofSolutions.aspx)
- [3\.7 More on the Wronskian](https://tutorial.math.lamar.edu/Classes/DE/Wronskian.aspx)
- [3\.8 Nonhomogeneous Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/NonhomogeneousDE.aspx)
- [3\.9 Undetermined Coefficients](https://tutorial.math.lamar.edu/Classes/DE/UndeterminedCoefficients.aspx)
- [3\.10 Variation of Parameters](https://tutorial.math.lamar.edu/Classes/DE/VariationofParameters.aspx)
- [3\.11 Mechanical Vibrations](https://tutorial.math.lamar.edu/Classes/DE/Vibrations.aspx)
- [4\. Laplace Transforms](https://tutorial.math.lamar.edu/Classes/DE/LaplaceIntro.aspx)
- [4\.1 The Definition](https://tutorial.math.lamar.edu/Classes/DE/LaplaceDefinition.aspx)
- [4\.2 Laplace Transforms](https://tutorial.math.lamar.edu/Classes/DE/LaplaceTransforms.aspx)
- [4\.3 Inverse Laplace Transforms](https://tutorial.math.lamar.edu/Classes/DE/InverseTransforms.aspx)
- [4\.4 Step Functions](https://tutorial.math.lamar.edu/Classes/DE/StepFunctions.aspx)
- [4\.5 Solving IVP's with Laplace Transforms](https://tutorial.math.lamar.edu/Classes/DE/IVPWithLaplace.aspx)
- [4\.6 Nonconstant Coefficient IVP's](https://tutorial.math.lamar.edu/Classes/DE/IVPWithNonConstantCoefficient.aspx)
- [4\.7 IVP's With Step Functions](https://tutorial.math.lamar.edu/Classes/DE/IVPWithStepFunction.aspx)
- [4\.8 Dirac Delta Function](https://tutorial.math.lamar.edu/Classes/DE/DiracDeltaFunction.aspx)
- [4\.9 Convolution Integrals](https://tutorial.math.lamar.edu/Classes/DE/ConvolutionIntegrals.aspx)
- [4\.10 Table Of Laplace Transforms](https://tutorial.math.lamar.edu/Classes/DE/Laplace_Table.aspx)
- [5\. Systems of DE's](https://tutorial.math.lamar.edu/Classes/DE/SystemsIntro.aspx)
- [5\.1 Review : Systems of Equations](https://tutorial.math.lamar.edu/Classes/DE/LA_Systems.aspx)
- [5\.2 Review : Matrices & Vectors](https://tutorial.math.lamar.edu/Classes/DE/LA_Matrix.aspx)
- [5\.3 Review : Eigenvalues & Eigenvectors](https://tutorial.math.lamar.edu/Classes/DE/LA_Eigen.aspx)
- [5\.4 Systems of Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/SystemsDE.aspx)
- [5\.5 Solutions to Systems](https://tutorial.math.lamar.edu/Classes/DE/SolutionsToSystems.aspx)
- [5\.6 Phase Plane](https://tutorial.math.lamar.edu/Classes/DE/PhasePlane.aspx)
- [5\.7 Real Eigenvalues](https://tutorial.math.lamar.edu/Classes/DE/RealEigenvalues.aspx)
- [5\.8 Complex Eigenvalues](https://tutorial.math.lamar.edu/Classes/DE/ComplexEigenvalues.aspx)
- [5\.9 Repeated Eigenvalues](https://tutorial.math.lamar.edu/Classes/DE/RepeatedEigenvalues.aspx)
- [5\.10 Nonhomogeneous Systems](https://tutorial.math.lamar.edu/Classes/DE/NonhomogeneousSystems.aspx)
- [5\.11 Laplace Transforms](https://tutorial.math.lamar.edu/Classes/DE/SystemsLaplace.aspx)
- [5\.12 Modeling](https://tutorial.math.lamar.edu/Classes/DE/SystemsModeling.aspx)
- [6\. Series Solutions to DE's](https://tutorial.math.lamar.edu/Classes/DE/SeriesIntro.aspx)
- [6\.1 Review : Power Series](https://tutorial.math.lamar.edu/Classes/DE/PowerSeries.aspx)
- [6\.2 Review : Taylor Series](https://tutorial.math.lamar.edu/Classes/DE/TaylorSeries.aspx)
- [6\.3 Series Solutions](https://tutorial.math.lamar.edu/Classes/DE/SeriesSolutions.aspx)
- [6\.4 Euler Equations](https://tutorial.math.lamar.edu/Classes/DE/EulerEquations.aspx)
- [7\. Higher Order Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/IntroHigherOrder.aspx)
- [7\.1 Basic Concepts for *n*th Order Linear Equations](https://tutorial.math.lamar.edu/Classes/DE/HOBasicConcepts.aspx)
- [7\.2 Linear Homogeneous Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/HOHomogeneousDE.aspx)
- [7\.3 Undetermined Coefficients](https://tutorial.math.lamar.edu/Classes/DE/HOUndeterminedCoeff.aspx)
- [7\.4 Variation of Parameters](https://tutorial.math.lamar.edu/Classes/DE/HOVariationOfParam.aspx)
- [7\.5 Laplace Transforms](https://tutorial.math.lamar.edu/Classes/DE/HOLaplaceTransforms.aspx)
- [7\.6 Systems of Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/HOSystems.aspx)
- [7\.7 Series Solutions](https://tutorial.math.lamar.edu/Classes/DE/HOSeries.aspx)
- [8\. Boundary Value Problems & Fourier Series](https://tutorial.math.lamar.edu/Classes/DE/IntroBVP.aspx)
- [8\.1 Boundary Value Problems](https://tutorial.math.lamar.edu/Classes/DE/BoundaryValueProblem.aspx)
- [8\.2 Eigenvalues and Eigenfunctions](https://tutorial.math.lamar.edu/Classes/DE/BVPEvals.aspx)
- [8\.3 Periodic Functions & Orthogonal Functions](https://tutorial.math.lamar.edu/Classes/DE/PeriodicOrthogonal.aspx)
- [8\.4 Fourier Sine Series](https://tutorial.math.lamar.edu/Classes/DE/FourierSineSeries.aspx)
- [8\.5 Fourier Cosine Series](https://tutorial.math.lamar.edu/Classes/DE/FourierCosineSeries.aspx)
- [8\.6 Fourier Series](https://tutorial.math.lamar.edu/Classes/DE/FourierSeries.aspx)
- [8\.7 Convergence of Fourier Series](https://tutorial.math.lamar.edu/Classes/DE/ConvergenceFourierSeries.aspx)
- [9\. Partial Differential Equations](https://tutorial.math.lamar.edu/Classes/DE/IntroPDE.aspx)
- [9\.1 The Heat Equation](https://tutorial.math.lamar.edu/Classes/DE/TheHeatEquation.aspx)
- [9\.2 The Wave Equation](https://tutorial.math.lamar.edu/Classes/DE/TheWaveEquation.aspx)
- [9\.3 Terminology](https://tutorial.math.lamar.edu/Classes/DE/PDETerminology.aspx)
- [9\.4 Separation of Variables](https://tutorial.math.lamar.edu/Classes/DE/SeparationofVariables.aspx)
- [9\.5 Solving the Heat Equation](https://tutorial.math.lamar.edu/Classes/DE/SolvingHeatEquation.aspx)
- [9\.6 Heat Equation with Non-Zero Temperature Boundaries](https://tutorial.math.lamar.edu/Classes/DE/HeatEqnNonZero.aspx)
- [9\.7 Laplace's Equation](https://tutorial.math.lamar.edu/Classes/DE/LaplacesEqn.aspx)
- [9\.8 Vibrating String](https://tutorial.math.lamar.edu/Classes/DE/VibratingString.aspx)
- [9\.9 Summary of Separation of Variables](https://tutorial.math.lamar.edu/Classes/DE/PDESummary.aspx)
- Extras
- [Algebra & Trig Review](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/AlgebraTrig.aspx)
- [1\. Algebra](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/AlgebraIntro.aspx)
- [1\.1 Exponents](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/Exponents.aspx)
- [1\.2 Absolute Value](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/AbsoluteValue.aspx)
- [1\.3 Radicals](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/Radicals.aspx)
- [1\.4 Rationalizing](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/Rationalizing.aspx)
- [1\.5 Functions](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/Functions.aspx)
- [1\.6 Multiplying Polynomials](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/MultPoly.aspx)
- [1\.7 Factoring](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/Factoring.aspx)
- [1\.8 Simplifying Rational Expressions](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SimpRatExp.aspx)
- [1\.9 Graphing and Common Graphs](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/Graphing.aspx)
- [1\.10 Solving Equations, Part I](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveEqnPtI.aspx)
- [1\.11 Solving Equations, Part II](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveEqnPtII.aspx)
- [1\.12 Solving Systems of Equations](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveSystems.aspx)
- [1\.13 Solving Inequalities](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveIneq.aspx)
- [1\.14 Absolute Value Equations and Inequalities](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveAbsValue.aspx)
- [2\. Trigonometry](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/TrigIntro.aspx)
- [2\.1 Trig Function Evaluation](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/TrigFunctions.aspx)
- [2\.2 Graphs of Trig Functions](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/TrigGraphs.aspx)
- [2\.3 Trig Formulas](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/TrigFormulas.aspx)
- [2\.4 Solving Trig Equations](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveTrigEqn.aspx)
- [2\.5 Inverse Trig Functions](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/InverseTrig.aspx)
- [3\. Exponentials & Logarithms](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/ExpLogIntro.aspx)
- [3\.1 Basic Exponential Functions](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/ExponentialFcns.aspx)
- [3\.2 Basic Logarithm Functions](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/LogarithmFcns.aspx)
- [3\.3 Logarithm Properties](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/LogProperties.aspx)
- [3\.4 Simplifying Logarithms](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SimpLogs.aspx)
- [3\.5 Solving Exponential Equations](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveExpEqn.aspx)
- [3\.6 Solving Logarithm Equations](https://tutorial.math.lamar.edu/Extras/AlgebraTrigReview/SolveLogEqn.aspx)
- [Common Math Errors](https://tutorial.math.lamar.edu/Extras/CommonErrors/CommonMathErrors.aspx)
- [1\. General Errors](https://tutorial.math.lamar.edu/Extras/CommonErrors/GeneralErrors.aspx)
- [2\. Algebra Errors](https://tutorial.math.lamar.edu/Extras/CommonErrors/AlgebraErrors.aspx)
- [3\. Trig Errors](https://tutorial.math.lamar.edu/Extras/CommonErrors/TrigErrors.aspx)
- [4\. Common Errors](https://tutorial.math.lamar.edu/Extras/CommonErrors/CommonErrors.aspx)
- [5\. Calculus Errors](https://tutorial.math.lamar.edu/Extras/CommonErrors/CalculusErrors.aspx)
- [Complex Number Primer](https://tutorial.math.lamar.edu/Extras/ComplexPrimer/ComplexNumbers.aspx)
- [1\. The Definition](https://tutorial.math.lamar.edu/Extras/ComplexPrimer/Definition.aspx)
- [2\. Arithmetic](https://tutorial.math.lamar.edu/Extras/ComplexPrimer/Arithmetic.aspx)
- [3\. Conjugate and Modulus](https://tutorial.math.lamar.edu/Extras/ComplexPrimer/ConjugateModulus.aspx)
- [4\. Polar and Exponential Forms](https://tutorial.math.lamar.edu/Extras/ComplexPrimer/Forms.aspx)
- [5\. Powers and Roots](https://tutorial.math.lamar.edu/Extras/ComplexPrimer/Roots.aspx)
- [How To Study Math](https://tutorial.math.lamar.edu/Extras/StudyMath/HowToStudyMath.aspx)
- [1\. General Tips](https://tutorial.math.lamar.edu/Extras/StudyMath/GeneralTips.aspx)
- [2\. Taking Notes](https://tutorial.math.lamar.edu/Extras/StudyMath/TakingNotes.aspx)
- [3\. Getting Help](https://tutorial.math.lamar.edu/Extras/StudyMath/GettingHelp.aspx)
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- [5\. Problem Solving](https://tutorial.math.lamar.edu/Extras/StudyMath/ProblemSolving.aspx)
- [6\. Studying For an Exam](https://tutorial.math.lamar.edu/Extras/StudyMath/StudyForExam.aspx)
- [7\. Taking an Exam](https://tutorial.math.lamar.edu/Extras/StudyMath/TakingExam.aspx)
- [8\. Learn From Your Errors](https://tutorial.math.lamar.edu/Extras/StudyMath/Errors.aspx)
- [Cheat Sheets & Tables](https://tutorial.math.lamar.edu/Extras/CheatSheets_Tables.aspx)
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Paul's Online Notes
[Home](https://tutorial.math.lamar.edu/ "Go to Main Page") / [Calculus III](https://tutorial.math.lamar.edu/Classes/CalcIII/CalcIII.aspx "Go to Book Introduction") / [Partial Derivatives](https://tutorial.math.lamar.edu/Classes/CalcIII/PartialDerivsIntro.aspx "Go to Chapter Introduction") / Higher Order Partial Derivatives
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### Section 13.4 : Higher Order Partial Derivatives
Just as we had higher order derivatives with functions of one variable we will also have higher order derivatives of functions of more than one variable. However, this time we will have more options since we do have more than one variable.
Consider the case of a function of two variables, \\(f\\left( {x,y} \\right)\\) since both of the first order partial derivatives are also functions of \\(x\\) and \\(y\\) we could in turn differentiate each with respect to \\(x\\) or \\(y\\). This means that for the case of a function of two variables there will be a total of four possible second order derivatives. Here they are and the notations that we’ll use to denote them.
\\\[\\begin{align\*}{\\left( {{f\_x}} \\right)\_x} & = {f\_{x\\,x}} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{\\partial f}}{{\\partial x}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial {x^2}}}\\\\ {\\left( {{f\_x}} \\right)\_y} & = {f\_{x\\,y}} = \\frac{\\partial }{{\\partial y}}\\left( {\\frac{{\\partial f}}{{\\partial x}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial y\\partial x}}\\\\ {\\left( {{f\_y}} \\right)\_x} & = {f\_{y\\,x}} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{\\partial f}}{{\\partial y}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial x\\partial y}}\\\\ {\\left( {{f\_y}} \\right)\_y} & = {f\_{y\\,y}} = \\frac{\\partial }{{\\partial y}}\\left( {\\frac{{\\partial f}}{{\\partial y}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial {y^2}}}\\end{align\*}\\\]
The second and third second order partial derivatives are often called mixed partial derivatives since we are taking derivatives with respect to more than one variable. Note as well that the order that we take the derivatives in is given by the notation for each these. If we are using the subscripting notation, *e.g.* \\({f\_{x\\,y}}\\), then we will differentiate from left to right. In other words, in this case, we will differentiate first with respect to \\(x\\) and then with respect to \\(y\\). With the fractional notation, *e.g.* \\(\\frac{{{\\partial ^2}f}}{{\\partial y\\partial x}}\\), it is the opposite. In these cases we differentiate moving along the denominator from right to left. So, again, in this case we differentiate with respect to \\(x\\) first and then \\(y\\).
Let’s take a quick look at an example.
Example 1 Find all the second order derivatives for \\(f\\left( {x,y} \\right) = \\cos \\left( {2x} \\right) - {x^2}{{\\bf{e}}^{5y}} + 3{y^2}\\).
Show Solution
We’ll first need the first order derivatives so here they are.
\\\[\\begin{align\*}{f\_x}\\left( {x,y} \\right) & = - 2\\sin \\left( {2x} \\right) - 2x{{\\bf{e}}^{5y}}\\\\ {f\_y}\\left( {x,y} \\right) & = - 5{x^2}{{\\bf{e}}^{5y}} + 6y\\end{align\*}\\\]
Now, let’s get the second order derivatives.
\\\[\\begin{align\*}{f\_{xx}} & = - 4\\cos \\left( {2x} \\right) - 2{{\\bf{e}}^{5y}}\\\\ {f\_{xy}} & = - 10x{{\\bf{e}}^{5y}}\\\\ {f\_{yx}} & = - 10x{{\\bf{e}}^{5y}}\\\\ {f\_{yy}} & = - 25{x^2}{{\\bf{e}}^{5y}} + 6\\end{align\*}\\\]
Notice that we dropped the \\(\\left( {x,y} \\right)\\) from the derivatives. This is fairly standard and we will be doing it most of the time from this point on. We will also be dropping it for the first order derivatives in most cases.
Now let’s also notice that, in this case, \\({f\_{xy}} = {f\_{yx}}\\). This is not by coincidence. If the function is “nice enough” this will always be the case. So, what’s “nice enough”? The following theorem tells us.
#### Clairaut’s Theorem
Suppose that \\(f\\) is defined on a disk \\(D\\) that contains the point \\(\\left( {a,b} \\right)\\). If the functions \\({f\_{xy}}\\) and \\({f\_{yx}}\\) are continuous on this disk then,
\\\[{f\_{xy}}\\left( {a,b} \\right) = {f\_{yx}}\\left( {a,b} \\right)\\\]
Now, do not get too excited about the disk business and the fact that we gave the theorem for a specific point. In pretty much every example in this class if the two mixed second order partial derivatives are continuous then they will be equal.
Example 2 Verify Clairaut’s Theorem for \\(f\\left( {x,y} \\right) = x{{\\bf{e}}^{ - {x^2}{y^2}}}\\).
Show Solution
We’ll first need the two first order derivatives.
\\\[\\begin{align\*}{f\_x}\\left( {x,y} \\right) & = {{\\bf{e}}^{ - {x^2}{y^2}}} - 2{x^2}{y^2}{{\\bf{e}}^{ - {x^2}{y^2}}}\\\\ {f\_y}\\left( {x,y} \\right) & = - 2y{x^3}{{\\bf{e}}^{ - {x^2}{y^2}}}\\end{align\*}\\\]
Now, compute the two mixed second order partial derivatives.
\\\[\\begin{align\*}{f\_{xy}}\\left( {x,y} \\right) & = - 2y{x^2}{{\\bf{e}}^{ - {x^2}{y^2}}} - 4{x^2}y{{\\bf{e}}^{ - {x^2}{y^2}}} + 4{x^4}{y^3}{{\\bf{e}}^{ - {x^2}{y^2}}} = - 6{x^2}y{{\\bf{e}}^{ - {x^2}{y^2}}} + 4{x^4}{y^3}{{\\bf{e}}^{ - {x^2}{y^2}}}\\\\ {f\_{yx}}\\left( {x,y} \\right) & = - 6y{x^2}{{\\bf{e}}^{ - {x^2}{y^2}}} + 4{y^3}{x^4}{{\\bf{e}}^{ - {x^2}{y^2}}}\\end{align\*}\\\]
Sure enough they are the same.
So far we have only looked at second order derivatives. There are, of course, higher order derivatives as well. Here are a couple of the third order partial derivatives of function of two variables.
\\\[\\begin{align\*}{f\_{x\\,y\\,x}} & = {\\left( {{f\_{xy}}} \\right)\_x} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{{\\partial ^2}f}}{{\\partial y\\partial x}}} \\right) = \\frac{{{\\partial ^3}f}}{{\\partial x\\partial y\\partial x}}\\\\ {f\_{y\\,x\\,x}} & = {\\left( {{f\_{yx}}} \\right)\_x} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{{\\partial ^2}f}}{{\\partial x\\partial y}}} \\right) = \\frac{{{\\partial ^3}f}}{{\\partial {x^2}\\partial y}}\\end{align\*}\\\]
Notice as well that for both of these we differentiate once with respect to \\(y\\) and twice with respect to \\(x\\). There is also another third order partial derivative in which we can do this, \\({f\_{x\\,x\\,y}}\\). There is an extension to Clairaut’s Theorem that says if all three of these are continuous then they should all be equal,
\\\[{f\_{x\\,x\\,y}} = {f\_{x\\,y\\,x}} = {f\_{y\\,x\\,x}}\\\]
To this point we’ve only looked at functions of two variables, but everything that we’ve done to this point will work regardless of the number of variables that we’ve got in the function and there are natural extensions to Clairaut’s theorem to all of these cases as well. For instance,
\\\[{f\_{x\\,z}}\\left( {x,y,z} \\right) = {f\_{z\\,x}}\\left( {x,y,z} \\right)\\\]
provided both of the derivatives are continuous.
In general, we can extend Clairaut’s theorem to any function and mixed partial derivatives. The only requirement is that in each derivative we differentiate with respect to each variable the same number of times. In other words, provided we meet the continuity condition, the following will be equal
\\\[{f\_{s\\,s\\,r\\,t\\,s\\,r\\,r}} = {f\_{t\\,r\\,s\\,r\\,s\\,s\\,r}}\\\]
because in each case we differentiate with respect to \\(t\\) once, \\(s\\) three times and \\(r\\) three times.
Let’s do a couple of examples with higher (well higher order than two anyway) order derivatives and functions of more than two variables.
Example 3 Find the indicated derivative for each of the following functions.
1. Find \\({f\_{x\\,x\\,y\\,z\\,z}}\\) for \\(f\\left( {x,y,z} \\right) = {z^3}{y^2}\\ln \\left( x \\right)\\)
2. Find \\(\\displaystyle \\frac{{{\\partial ^3}f}}{{\\partial y\\partial {x^2}}}\\) for \\(f\\left( {x,y} \\right) = {{\\bf{e}}^{xy}}\\)
Show All Solutions Hide All Solutions
a Find \\({f\_{x\\,x\\,y\\,z\\,z}}\\) for \\(f\\left( {x,y,z} \\right) = {z^3}{y^2}\\ln \\left( x \\right)\\) Show Solution
In this case remember that we differentiate from left to right. Here are the derivatives for this part.
\\\[{f\_x} = \\frac{{{z^3}{y^2}}}{x}\\\] \\\[{f\_{xx}} = - \\frac{{{z^3}{y^2}}}{{{x^2}}}\\\] \\\[{f\_{xxy}} = - \\frac{{2{z^3}y}}{{{x^2}}}\\\] \\\[{f\_{xxyz}} = - \\frac{{6{z^2}y}}{{{x^2}}}\\\] \\\[{f\_{xxyzz}} = - \\frac{{12zy}}{{{x^2}}}\\\]
b Find \\(\\displaystyle \\frac{{{\\partial ^3}f}}{{\\partial y\\partial {x^2}}}\\) for \\(f\\left( {x,y} \\right) = {{\\bf{e}}^{xy}}\\) Show Solution
Here we differentiate from right to left. Here are the derivatives for this function.
\\\[\\frac{{\\partial f}}{{\\partial x}} = y{{\\bf{e}}^{xy}}\\\] \\\[\\frac{{{\\partial ^2}f}}{{\\partial {x^2}}} = {y^2}{{\\bf{e}}^{xy}}\\\] \\\[\\frac{{{\\partial ^3}f}}{{\\partial y\\partial {x^2}}} = 2y{{\\bf{e}}^{xy}} + x{y^2}{{\\bf{e}}^{xy}}\\\]
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### Section 13.4 : Higher Order Partial Derivatives
Just as we had higher order derivatives with functions of one variable we will also have higher order derivatives of functions of more than one variable. However, this time we will have more options since we do have more than one variable.
Consider the case of a function of two variables, \\(f\\left( {x,y} \\right)\\) since both of the first order partial derivatives are also functions of \\(x\\) and \\(y\\) we could in turn differentiate each with respect to \\(x\\) or \\(y\\). This means that for the case of a function of two variables there will be a total of four possible second order derivatives. Here they are and the notations that we’ll use to denote them.
\\\[\\begin{align\*}{\\left( {{f\_x}} \\right)\_x} & = {f\_{x\\,x}} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{\\partial f}}{{\\partial x}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial {x^2}}}\\\\ {\\left( {{f\_x}} \\right)\_y} & = {f\_{x\\,y}} = \\frac{\\partial }{{\\partial y}}\\left( {\\frac{{\\partial f}}{{\\partial x}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial y\\partial x}}\\\\ {\\left( {{f\_y}} \\right)\_x} & = {f\_{y\\,x}} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{\\partial f}}{{\\partial y}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial x\\partial y}}\\\\ {\\left( {{f\_y}} \\right)\_y} & = {f\_{y\\,y}} = \\frac{\\partial }{{\\partial y}}\\left( {\\frac{{\\partial f}}{{\\partial y}}} \\right) = \\frac{{{\\partial ^2}f}}{{\\partial {y^2}}}\\end{align\*}\\\]
The second and third second order partial derivatives are often called mixed partial derivatives since we are taking derivatives with respect to more than one variable. Note as well that the order that we take the derivatives in is given by the notation for each these. If we are using the subscripting notation, *e.g.* \\({f\_{x\\,y}}\\), then we will differentiate from left to right. In other words, in this case, we will differentiate first with respect to \\(x\\) and then with respect to \\(y\\). With the fractional notation, *e.g.* \\(\\frac{{{\\partial ^2}f}}{{\\partial y\\partial x}}\\), it is the opposite. In these cases we differentiate moving along the denominator from right to left. So, again, in this case we differentiate with respect to \\(x\\) first and then \\(y\\).
Let’s take a quick look at an example.
Example 1 Find all the second order derivatives for \\(f\\left( {x,y} \\right) = \\cos \\left( {2x} \\right) - {x^2}{{\\bf{e}}^{5y}} + 3{y^2}\\).
Show Solution
We’ll first need the first order derivatives so here they are.
\\\[\\begin{align\*}{f\_x}\\left( {x,y} \\right) & = - 2\\sin \\left( {2x} \\right) - 2x{{\\bf{e}}^{5y}}\\\\ {f\_y}\\left( {x,y} \\right) & = - 5{x^2}{{\\bf{e}}^{5y}} + 6y\\end{align\*}\\\]
Now, let’s get the second order derivatives.
\\\[\\begin{align\*}{f\_{xx}} & = - 4\\cos \\left( {2x} \\right) - 2{{\\bf{e}}^{5y}}\\\\ {f\_{xy}} & = - 10x{{\\bf{e}}^{5y}}\\\\ {f\_{yx}} & = - 10x{{\\bf{e}}^{5y}}\\\\ {f\_{yy}} & = - 25{x^2}{{\\bf{e}}^{5y}} + 6\\end{align\*}\\\]
Notice that we dropped the \\(\\left( {x,y} \\right)\\) from the derivatives. This is fairly standard and we will be doing it most of the time from this point on. We will also be dropping it for the first order derivatives in most cases.
Now let’s also notice that, in this case, \\({f\_{xy}} = {f\_{yx}}\\). This is not by coincidence. If the function is “nice enough” this will always be the case. So, what’s “nice enough”? The following theorem tells us.
#### Clairaut’s Theorem
Suppose that \\(f\\) is defined on a disk \\(D\\) that contains the point \\(\\left( {a,b} \\right)\\). If the functions \\({f\_{xy}}\\) and \\({f\_{yx}}\\) are continuous on this disk then,
\\\[{f\_{xy}}\\left( {a,b} \\right) = {f\_{yx}}\\left( {a,b} \\right)\\\]
Now, do not get too excited about the disk business and the fact that we gave the theorem for a specific point. In pretty much every example in this class if the two mixed second order partial derivatives are continuous then they will be equal.
Example 2 Verify Clairaut’s Theorem for \\(f\\left( {x,y} \\right) = x{{\\bf{e}}^{ - {x^2}{y^2}}}\\).
Show Solution
We’ll first need the two first order derivatives.
\\\[\\begin{align\*}{f\_x}\\left( {x,y} \\right) & = {{\\bf{e}}^{ - {x^2}{y^2}}} - 2{x^2}{y^2}{{\\bf{e}}^{ - {x^2}{y^2}}}\\\\ {f\_y}\\left( {x,y} \\right) & = - 2y{x^3}{{\\bf{e}}^{ - {x^2}{y^2}}}\\end{align\*}\\\]
Now, compute the two mixed second order partial derivatives.
\\\[\\begin{align\*}{f\_{xy}}\\left( {x,y} \\right) & = - 2y{x^2}{{\\bf{e}}^{ - {x^2}{y^2}}} - 4{x^2}y{{\\bf{e}}^{ - {x^2}{y^2}}} + 4{x^4}{y^3}{{\\bf{e}}^{ - {x^2}{y^2}}} = - 6{x^2}y{{\\bf{e}}^{ - {x^2}{y^2}}} + 4{x^4}{y^3}{{\\bf{e}}^{ - {x^2}{y^2}}}\\\\ {f\_{yx}}\\left( {x,y} \\right) & = - 6y{x^2}{{\\bf{e}}^{ - {x^2}{y^2}}} + 4{y^3}{x^4}{{\\bf{e}}^{ - {x^2}{y^2}}}\\end{align\*}\\\]
Sure enough they are the same.
So far we have only looked at second order derivatives. There are, of course, higher order derivatives as well. Here are a couple of the third order partial derivatives of function of two variables.
\\\[\\begin{align\*}{f\_{x\\,y\\,x}} & = {\\left( {{f\_{xy}}} \\right)\_x} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{{\\partial ^2}f}}{{\\partial y\\partial x}}} \\right) = \\frac{{{\\partial ^3}f}}{{\\partial x\\partial y\\partial x}}\\\\ {f\_{y\\,x\\,x}} & = {\\left( {{f\_{yx}}} \\right)\_x} = \\frac{\\partial }{{\\partial x}}\\left( {\\frac{{{\\partial ^2}f}}{{\\partial x\\partial y}}} \\right) = \\frac{{{\\partial ^3}f}}{{\\partial {x^2}\\partial y}}\\end{align\*}\\\]
Notice as well that for both of these we differentiate once with respect to \\(y\\) and twice with respect to \\(x\\). There is also another third order partial derivative in which we can do this, \\({f\_{x\\,x\\,y}}\\). There is an extension to Clairaut’s Theorem that says if all three of these are continuous then they should all be equal,
\\\[{f\_{x\\,x\\,y}} = {f\_{x\\,y\\,x}} = {f\_{y\\,x\\,x}}\\\]
To this point we’ve only looked at functions of two variables, but everything that we’ve done to this point will work regardless of the number of variables that we’ve got in the function and there are natural extensions to Clairaut’s theorem to all of these cases as well. For instance,
\\\[{f\_{x\\,z}}\\left( {x,y,z} \\right) = {f\_{z\\,x}}\\left( {x,y,z} \\right)\\\]
provided both of the derivatives are continuous.
In general, we can extend Clairaut’s theorem to any function and mixed partial derivatives. The only requirement is that in each derivative we differentiate with respect to each variable the same number of times. In other words, provided we meet the continuity condition, the following will be equal
\\\[{f\_{s\\,s\\,r\\,t\\,s\\,r\\,r}} = {f\_{t\\,r\\,s\\,r\\,s\\,s\\,r}}\\\]
because in each case we differentiate with respect to \\(t\\) once, \\(s\\) three times and \\(r\\) three times.
Let’s do a couple of examples with higher (well higher order than two anyway) order derivatives and functions of more than two variables.
Example 3 Find the indicated derivative for each of the following functions.
1. Find \\({f\_{x\\,x\\,y\\,z\\,z}}\\) for \\(f\\left( {x,y,z} \\right) = {z^3}{y^2}\\ln \\left( x \\right)\\)
2. Find \\(\\displaystyle \\frac{{{\\partial ^3}f}}{{\\partial y\\partial {x^2}}}\\) for \\(f\\left( {x,y} \\right) = {{\\bf{e}}^{xy}}\\)
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a Find \\({f\_{x\\,x\\,y\\,z\\,z}}\\) for \\(f\\left( {x,y,z} \\right) = {z^3}{y^2}\\ln \\left( x \\right)\\) Show Solution
In this case remember that we differentiate from left to right. Here are the derivatives for this part.
\\\[{f\_x} = \\frac{{{z^3}{y^2}}}{x}\\\] \\\[{f\_{xx}} = - \\frac{{{z^3}{y^2}}}{{{x^2}}}\\\] \\\[{f\_{xxy}} = - \\frac{{2{z^3}y}}{{{x^2}}}\\\] \\\[{f\_{xxyz}} = - \\frac{{6{z^2}y}}{{{x^2}}}\\\] \\\[{f\_{xxyzz}} = - \\frac{{12zy}}{{{x^2}}}\\\]
b Find \\(\\displaystyle \\frac{{{\\partial ^3}f}}{{\\partial y\\partial {x^2}}}\\) for \\(f\\left( {x,y} \\right) = {{\\bf{e}}^{xy}}\\) Show Solution
Here we differentiate from right to left. Here are the derivatives for this function.
\\\[\\frac{{\\partial f}}{{\\partial x}} = y{{\\bf{e}}^{xy}}\\\] \\\[\\frac{{{\\partial ^2}f}}{{\\partial {x^2}}} = {y^2}{{\\bf{e}}^{xy}}\\\] \\\[\\frac{{{\\partial ^3}f}}{{\\partial y\\partial {x^2}}} = 2y{{\\bf{e}}^{xy}} + x{y^2}{{\\bf{e}}^{xy}}\\\] |
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