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URLhttps://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-3-8.php
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Meta TitlePartial Derivatives
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Higher Order Derivatives Since f x and f y are functions of x and y , we can take derivatives of f x and f y to get second derivatives .Ā  There are four such second derivatives, since each time we can differentiate with respect to x or y .Ā  Each of these second derivatives has multiple notations, and we have listed some of them. Notation for second partial derivatives ( f x ) x = f x x = f 11 = āˆ‚ āˆ‚ x ( āˆ‚ f āˆ‚ x ) = āˆ‚ 2 f āˆ‚ x 2 = āˆ‚ 2 z āˆ‚ x 2 ( f x ) y = f x y = f 12 = āˆ‚ āˆ‚ y ( āˆ‚ f āˆ‚ x ) = āˆ‚ 2 f āˆ‚ y āˆ‚ x = āˆ‚ 2 z āˆ‚ y āˆ‚ x ( f y ) x = f y x = f 21 = āˆ‚ āˆ‚ x ( āˆ‚ f āˆ‚ y ) = āˆ‚ 2 f āˆ‚ x āˆ‚ y = āˆ‚ 2 z āˆ‚ x āˆ‚ y ( f y ) y = f y y = f 22 = āˆ‚ āˆ‚ y ( āˆ‚ f āˆ‚ y ) = āˆ‚ 2 f āˆ‚ y 2 = āˆ‚ 2 z āˆ‚ y 2 A nice result regarding second partial derivatives is Clairaut's Theorem , which tells us that the mixed variable partial derivatives are equal. Clairaut's Theorem If f x y and f y x are both defined and continuous in a region containing the point ( a , b ) , then f x y ( a , b ) = f y x ( a , b ) . A consequence of this theorem is that we don't need to keep track of the order in which we take derivatives.Ā  Example 1 :Ā  Let f ( x , y ) = 3 x 2 āˆ’ 4 y 3 āˆ’ 7 x 2 y 3 .Ā  Previously, we determined that f x = 6 x āˆ’ 14 x y 3 and f y = āˆ’ 12 y 2 āˆ’ 21 x 2 y 2 .Ā  We have four second derivatives, but as Clairaut's Theorem tells us, f x y = f y x , so we really only need to compute three of them (we do all four to illustrate the theorem). Solution 1 : ( f x ) x = āˆ‚ āˆ‚ x f x = āˆ‚ āˆ‚ x ( 6 x āˆ’ 14 x y 3 ) = 6 āˆ’ 14 y 3 ( f x ) y = āˆ‚ āˆ‚ y f x = āˆ‚ āˆ‚ y ( 6 x āˆ’ 14 x y 3 ) = 0 āˆ’ 42 x y 2 = āˆ’ 42 x y 2 ( f y ) x = āˆ‚ āˆ‚ x f y = āˆ‚ āˆ‚ x ( 12 y 2 āˆ’ 21 x 2 y 2 ) = 0 āˆ’ 42 x y 2 = āˆ’ 42 x y 2 ( f y ) y = āˆ‚ āˆ‚ y f y = āˆ‚ āˆ‚ y ( 12 y 2 āˆ’ 21 x 2 y 2 ) = 24 y āˆ’ 42 x 2 y Higher partial derivatives and Clairaut's theorem are explained in the following video.
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| | | | |---|---|---| | ![](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/logo.jpg) [Home](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/index.html) Integration by Parts [Integration by Parts](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-1-2.php) [Examples](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-1-3.php) [Integration by Parts with a definite integral](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-1-5.php) [Going in Circles](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-1-7.php) [Tricks of the Trade](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-1-8.php) Integrals of Trig Functions [Antiderivatives of Basic Trigonometric Functions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-2-2.php) [Product of Sines and Cosines (mixed even and odd powers or only odd powers)](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-2-3.php) [Product of Sines and Cosines (only even powers)](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-2-6.php) [Product of Secants and Tangents](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-2-8.php) [Other Cases](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-2-12.php) Trig Substitutions [How Trig Substitution Works](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-3-2.php) [Summary of trig substitution options](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-3-3.php) [Examples](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-3-5.php) [Completing the Square](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-3-9.php) Partial Fractions [Introduction to Partial Fractions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-4-2.php) [Linear Factors](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-4-3.php) [Irreducible Quadratic Factors](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-4-6.php) [Improper Rational Functions and Long Division](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-4-8.php) [Summary](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-4-9.php) Strategies of Integration [Substitution](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-5-2.php) [Integration by Parts](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-5-3.php) [Trig Integrals](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-5-4.php) [Trig Substitutions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-5-5.php) [Partial Fractions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-5-6.php) Improper Integrals [Type 1 - Improper Integrals with Infinite Intervals of Integration](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-8-2.php) [Type 2 - Improper Integrals with Discontinuous Integrands](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-8-4.php) [Comparison Tests for Convergence](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM7-8-7.php) Modeling with Differential Equations [Introduction](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-1-2.php) [Separable Equations](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-1-3.php) [A Second Order Problem](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-1-4.php) Euler's Method and Direction Fields [Euler's Method (follow your nose)](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-2-2.php) [Direction Fields](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-2-4.php) [Euler's method revisited](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-2-5.php) Separable Equations [The Simplest Differential Equations](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-3-2.php) [Separable differential equations](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-3-3.php) [Mixing and Dilution](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-3-4.php) Models of Growth [Exponential Growth and Decay](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-4-2.php) [The Zombie Apocalypse (Logistic Growth)](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-4-3.php) Linear Equations [Linear ODEs: Working an Example](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-5-2.php) [The Solution in General](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-5-3.php) [Saving for Retirement](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM9-5-4.php) Parametrized Curves [Three kinds of functions, three kinds of curves](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-1-2.php) [The Cycloid](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-1-3.php) [Visualizing Parametrized Curves](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-1-4.php) [Tracing Circles and Ellipses](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-1-5.php) [Lissajous Figures](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-1-6.php) Calculus with Parametrized Curves [Video: Slope and Area](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-2-2.php) [Video: Arclength and Surface Area](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-2-3.php) [Summary and Simplifications](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-2-4.php) [Higher Derivatives](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-2-5.php) Polar Coordinates [Definitions of Polar Coordinates](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-3-2.php) [Graphing polar functions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-3-3.php) [Video: Computing Slopes of Tangent Lines](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-3-4.php) Areas and Lengths of Polar Curves [Area Inside a Polar 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Polar Coordinates](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM10-6-4.php) Infinite Sequences [Approximate Versus Exact Answers](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-1-2.php) [Examples of Infinite Sequences](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-1-3.php) [Limit Laws for Sequences](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-1-6.php) [Theorems for and Examples of Computing Limits of Sequences](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-1-7.php) [Monotonic Covergence](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-1-10.php) Infinite Series [Introduction](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-2-2.php) [Geometric Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-2-4.php) [Limit Laws for Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-2-5.php) [Test for Divergence and Other Theorems](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-2-7.php) [Telescoping Sums](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-2-9.php) Integral Test [Preview of Coming Attractions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-3-2.php) [The Integral Test](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-3-3.php) [Estimates for the Value of the Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-3-5.php) Comparison Tests [The Basic Comparison Test](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-4-2.php) [The Limit Comparison Test](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-4-4.php) Convergence of Series with Negative Terms [Introduction, Alternating Series,and the AS Test](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-5-2.php) [Absolute Convergence](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-5-4.php) [Rearrangements](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-5-8.php) The Ratio and Root Tests [The Ratio Test](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-6-2.php) [The Root Test](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-6-4.php) [Examples](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-6-6.php) Strategies for testing Series [Strategy to Test Series and a Review of Tests](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-7-2.php) [Examples, Part 1](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-7-3.php) [Examples, Part 2](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-7-4.php) Power Series [Radius and Interval of Convergence](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-8-2.php) [Finding the Interval of Convergence](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-8-3.php) [Power Series Centered at x=a x = a](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-8-6.php) Representing Functions as Power Series [Functions as Power Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-9-2.php) [Derivatives and Integrals of Power Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-9-4.php) [Applications and Examples](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-9-5.php) Taylor and Maclaurin Series [The Formula for Taylor Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-10-2.php) [Taylor Series for Common Functions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-10-4.php) [Adding, Multiplying, and Dividing Power Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-10-7.php) [Miscellaneous Useful Facts](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-10-8.php) Applications of Taylor Polynomials [Taylor Polynomials](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-11-2.php) [When Functions Are Equal to Their Taylor Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-11-5.php) [When a Function Does Not Equal Its Taylor Series](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-11-7.php) [Other Uses of Taylor Polynomials](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM11-11-8.php) Functions of 2 and 3 variables [Functions of several variables](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-1-2.php) [Limits and continuity](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-1-4.php) Partial Derivatives [One variable at a time (yet again)](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-3-2.php) [Definitions and Examples](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-3-3.php) [An Example from DNA](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-3-5.php) [Geometry of partial derivatives](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-3-6.php) [Higher Derivatives](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-3-8.php) [Differentials and Taylor Expansions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-3-10.php) Differentiability and the Chain Rule [Differentiability](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-5-2.php) [The First Case of the Chain Rule](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-5-3.php) [Chain Rule, General Case](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-5-4.php) [Video: Worked problems](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM14-5-5.php) Multiple Integrals [General Setup and Review of 1D Integrals](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-1-2.php) [What is a Double Integral?](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-1-3.php) [Volumes as Double Integrals](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-1-4.php) Iterated Integrals over Rectangles [How To Compute Iterated Integrals](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-2-2.php) [Examples of Iterated Integrals](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-2-3.php) [Fubini's Theorem](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-2-7.php) [Summary and an Important Example](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-2-8.php) Double Integrals over General Regions [Type I and Type II regions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-3-2.php) [Examples 1-4](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-3-3.php) [Examples 5-7](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-3-4.php) [Swapping the Order of Integration](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-3-6.php) [Area and Volume Revisited](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-3-7.php) Double integrals in polar coordinates [dA = r dr (d theta)](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-4-2.php) [Examples](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-4-3.php) Multiple integrals in physics [Double integrals in physics](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-5a-2.php) [Triple integrals in physics](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-5a-3.php) Integrals in Probability and Statistics [Single integrals in probability](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-5b-2.php) [Double integrals in probability](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-5b-3.php) Change of Variables [Review: Change of variables in 1 dimension](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-10-2.php) [Mappings in 2 dimensions](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-10-3.php) [Jacobians](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-10-4.php) [Examples](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-10-5.php) [Bonus: Cylindrical and spherical coordinates](https://web.ma.utexas.edu/users/m408s/m408d/CurrentWeb/LM15-10-8.php) | | | | | | | | **Notation for second partial derivatives** (fx)x\=fxx\=f11\=āˆ‚āˆ‚x(āˆ‚fāˆ‚x)\=āˆ‚2fāˆ‚x2\=āˆ‚2zāˆ‚x2 ( f x ) x \= f x x \= f 11 \= āˆ‚ āˆ‚ x ( āˆ‚ f āˆ‚ x ) \= āˆ‚ 2 f āˆ‚ x 2 \= āˆ‚ 2 z āˆ‚ x 2 (fx)y\=fxy\=f12\=āˆ‚āˆ‚y(āˆ‚fāˆ‚x)\=āˆ‚2fāˆ‚yāˆ‚x\=āˆ‚2zāˆ‚yāˆ‚x ( f x ) y \= f x y \= f 12 \= āˆ‚ āˆ‚ y ( āˆ‚ f āˆ‚ x ) \= āˆ‚ 2 f āˆ‚ y āˆ‚ x \= āˆ‚ 2 z āˆ‚ y āˆ‚ x (fy)x\=fyx\=f21\=āˆ‚āˆ‚x(āˆ‚fāˆ‚y)\=āˆ‚2fāˆ‚xāˆ‚y\=āˆ‚2zāˆ‚xāˆ‚y ( f y ) x \= f y x \= f 21 \= āˆ‚ āˆ‚ x ( āˆ‚ f āˆ‚ y ) \= āˆ‚ 2 f āˆ‚ x āˆ‚ y \= āˆ‚ 2 z āˆ‚ x āˆ‚ y (fy)y\=fyy\=f22\=āˆ‚āˆ‚y(āˆ‚fāˆ‚y)\=āˆ‚2fāˆ‚y2\=āˆ‚2zāˆ‚y2 ( f y ) y \= f y y \= f 22 \= āˆ‚ āˆ‚ y ( āˆ‚ f āˆ‚ y ) \= āˆ‚ 2 f āˆ‚ y 2 \= āˆ‚ 2 z āˆ‚ y 2 | | | | | | | | **Clairaut's Theorem** If fxy f x y and fyx f y x are both defined and continuous in a region containing the point (a,b) ( a , b ), then fxy(a,b)\=fyx(a,b). f x y ( a , b ) \= f y x ( a , b ) . | | |
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### Higher Order Derivatives Since f x and f y are functions of x and y, **we can take derivatives of f x and f y to get second derivatives**. There are four such second derivatives, since each time we can differentiate with respect to x or y. Each of these second derivatives has multiple notations, and we have listed some of them. **Notation for second partial derivatives** ( f x ) x \= f x x \= f 11 \= āˆ‚ āˆ‚ x ( āˆ‚ f āˆ‚ x ) \= āˆ‚ 2 f āˆ‚ x 2 \= āˆ‚ 2 z āˆ‚ x 2 ( f x ) y \= f x y \= f 12 \= āˆ‚ āˆ‚ y ( āˆ‚ f āˆ‚ x ) \= āˆ‚ 2 f āˆ‚ y āˆ‚ x \= āˆ‚ 2 z āˆ‚ y āˆ‚ x ( f y ) x \= f y x \= f 21 \= āˆ‚ āˆ‚ x ( āˆ‚ f āˆ‚ y ) \= āˆ‚ 2 f āˆ‚ x āˆ‚ y \= āˆ‚ 2 z āˆ‚ x āˆ‚ y ( f y ) y \= f y y \= f 22 \= āˆ‚ āˆ‚ y ( āˆ‚ f āˆ‚ y ) \= āˆ‚ 2 f āˆ‚ y 2 \= āˆ‚ 2 z āˆ‚ y 2 A nice result regarding second partial derivatives is **Clairaut's Theorem**, which tells us that the mixed variable partial derivatives are equal. **Clairaut's Theorem** If f x y and f y x are both defined and continuous in a region containing the point ( a , b ) , then f x y ( a , b ) \= f y x ( a , b ) . A consequence of this theorem is that we don't need to keep track of the order in which we take derivatives. **Example 1**: Let f ( x , y ) \= 3 x 2 āˆ’ 4 y 3 āˆ’ 7 x 2 y 3. Previously, we determined that f x \= 6 x āˆ’ 14 x y 3 and f y \= āˆ’ 12 y 2 āˆ’ 21 x 2 y 2. We have four second derivatives, but as Clairaut's Theorem tells us, f x y \= f y x, so we really only need to compute three of them (we do all four to illustrate the theorem). **Solution 1**: ( f x ) x \= āˆ‚ āˆ‚ x f x \= āˆ‚ āˆ‚ x ( 6 x āˆ’ 14 x y 3 ) \= 6 āˆ’ 14 y 3 ( f x ) y \= āˆ‚ āˆ‚ y f x \= āˆ‚ āˆ‚ y ( 6 x āˆ’ 14 x y 3 ) \= 0 āˆ’ 42 x y 2 \= āˆ’ 42 x y 2 ( f y ) x \= āˆ‚ āˆ‚ x f y \= āˆ‚ āˆ‚ x ( 12 y 2 āˆ’ 21 x 2 y 2 ) \= 0 āˆ’ 42 x y 2 \= āˆ’ 42 x y 2 ( f y ) y \= āˆ‚ āˆ‚ y f y \= āˆ‚ āˆ‚ y ( 12 y 2 āˆ’ 21 x 2 y 2 ) \= 24 y āˆ’ 42 x 2 y Higher partial derivatives and Clairaut's theorem are explained in the following video.
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