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| Boilerpipe Text | The partial derivative of a function (in two or more variables) is its derivative with respect to one of the variables keeping all the other variables as constants. The process of calculating partial derivative is as same as that of an ordinary derivative except we consider the other variables than the variable with respect to which we are differentiating as constants.
Let us learn more about how to calculate partial derivatives of different orders along with examples.
1.
What is Partial Derivative?
2.
Partial Derivative Symbol
3.
How to Calculate Partial Derivatives?
4.
Partial Derivatives of Different Orders
5.
Partial Differentiation Formulas
6.
FAQs on Partial Derivative
The
partial derivative
of a multivariable function, say z = f(x, y), is its
derivative
with respect to one of the variables, x or y in this case, where the other variables are treated as constants. For example,
for finding the partial derivative of f(x, y) with respect to x (which is represented by βf / βx), y is treated as constant and
for finding the partial derivative of f(x, y) with respect to y (which is represented by βf / βy), x is treated as constant
Note that we are not considering all the variables as variables while doing partial differentiation (instead, we are considering only one variable as a variable at a time) and hence the name "partial". The limit definition of a partial derivative looks very similar to the
limit
definition of the derivative. We can find the partial derivatives using the following limit formulas:
βf / βx = lim
h β 0
[ f(x + h, y) - f(x, y) ] / h
βf / βy = lim
h β 0
[ f(x, y + h) - f(x, y) ] / h
These formulas resemble the derivative definition using the first principle.
Example of Partial Derivative
If f (x, y) = xy, then find the partial derivative βf / βx.
Solution:
βf / βx = lim
h β 0
[ f(x + h, y) - f(x, y) ] / h
= lim
h β 0
[ (x + h)y - xy ] / h
= lim
h β 0
[xy + hy - xy] / h
= lim
h β 0
[hy]/h
= lim
h β 0
y
= y
Therefore, βf / βx = y.
We know that the ordinary derivative of a function y = f(x) is denoted by one of the notations dy/dx, d/dx (y), d/dx (f(x)), f '(x), etc. For representing a partial derivative we use the symbol "β" instead of "d". We pronounce "β" to be "doh" but it has some other names like "partial", "del", "partial dee", "dee", "Jacobi's delta", etc. If z = f(x, y) is a function in two variables then
βf / βx is the partial derivative of f with respect to x
βf / βy is the partial derivative of f with respect to y
Just like how we have different
symbols
of ordinary derivatives, we have different notations for partial derivatives as well. For example, βf / βx can be written as f
x
, f
x
', D
x
f, β / βx (f), β
x
f, β / βx [f(x, y)], βz / βx, etc.
We have already seen that the limit definitions are used to find the partial derivatives. But using the
limit formula
and computing the limit is not always easy. Thus, we have another method to calculate partial derivatives that follow right from its definition. In this method, if z = f(x, y) is the function, then we can compute the partial derivatives using the following steps:
Step 1
: Identify the variable with respect to which we have to find the partial derivative.
Step 2:
Except for the variable found in
Step 1
, treat all the other variables as constants.
Step 3:
Differentiate the function just using the rules of ordinary differentiation.
Wait! Read
Step 3
again. Yes, the rules of ordinary differentiation are as same as that of partial differentiation. In partial differentiation, just treating variables is different, that's it!
Example:
Let us solve the same above example (If f (x, y) = xy, then find the partial derivative βf / βx) using the above steps.
Solution:
We have to find βf / βx. It means, we have to find the partial derivative of f with respect to x. So we treat y as constant. Thus, we can write 'y' outside the derivative (as in ordinary differentiation, we have a rule that says d/dx (c y) = c dy/dx, where 'c' is a constant). Thus,
βf / βx = β / βx (xy)
= y β / βx (x)
= y (1) (Using
power rule
, d/dx (x) = 1)
= y
We have got the same answer as we got using the limit definition.
We have derivatives like first-order derivatives (like dy/dx),
second-order derivatives
(like d
2
y/dx
2
), etc in ordinary derivatives. Likewise, we have first-order, second-order, and higher-order derivatives in partial derivatives also.
First Order Partial Derivatives
If z = f(x, y) is a function in two variables, then it can have two first-order partial derivatives, namely βf / βx and βf / βy.
Example:
If z = x
2
+ y
2
, find all the first order partial derivatives.
Solution:
f
x
= βf / βx = β / βx (x
2
+ y
2
)
= β / βx (x
2
) + β / βx (y
2
)
= 2x + 0 (as y is a constant)
= 2x
f
y
= βf / βy = β / βy (x
2
+ y
2
)
= β / βy (x
2
) + β / βy (y
2
)
= 0 + 2y (as x is a constant)
= 2y
Second Order Partial Derivatives
The second-order partial derivative is obtained by differentiating the function with respect to the indicated variables successively one after the other. If z = f(x, y) is a function in two variables, then it can have four second-order partial derivatives, namely β
2
f / βx
2
, β
2
f / βy
2
, β
2
f / βx βy and β
2
f / βy ββxβββ. To find them, we can first differentiate the function partially with the latter variable, and then partially differentiate the result with respect to the former variable. i.e.,
f
xx
= β
2
f / βx
2
= β / βx (βf / βx) = β / βx (f
x
)
f
yy
= β
2
f / βy
2
= β / βy (βf / βy) = β / βx (f
y
)
f
yx
= β
2
f / βx βy = β / βx (βf / βy) = β / βx (f
y
)
f
xy
= β
2
f / βy βx = β / βy (βf / βx) = β / βy (f
x
)
Observe the notations f
yx
and f
xy
. The order of variables in each subscript indicate the order of partial differentiation. For example, f
yx
means to partially differentiate with respect to y first and then with respect to x, and this is same as β
2
f / βx βy.
Example:
If z = x
2
+ y
2
, find all the second order partial derivatives.
Solution:
In the above example, we have already found that f
x
= 2x and f
y
= 2y.
Now, f
xx
= β / βx (f
x
) = β / βx (2x) = 2
f
yy
= β / βy (f
y
) = β / βy (2y) = 2
f
yx
= β / βx (f
y
) = β / βx (2y) = 0
f
xy
= β / βy (f
x
) = β / βy (2x) = 0
Now that f
yx
= f
xy
. Thus, the order of partial differentiation doesn't matter.
The process of finding partial derivatives is known as Partial Differentiation. To find the first-order partial derivatives (as discussed earlier) of a function z = f(x, y) we use the following limit formulas:
βf / βx = lim
h β 0
[ f(x + h, y) - f(x, y) ] / h
βf / βy = lim
h β 0
[ f(x, y + h) - f(x, y) ] / h
But instead of using these formulas, just treating all the other variables than the variable with respect to which we are partially differentiating as constants would make the process of partial differentiation very easier. In this process, we just use the same
rules as ordinary differentiation
and among them, the important rules are as follows:
Power Rule
The power rule of differentiation says d/dx (x
n
) = n x
n-1
. The same rule can be applied in partial derivatives also.
Example:
β / βx (x
2
y) = y β / βx (x
2
) = y (2x) = 2xy.
Product Rule
The
product rule
of ordinary differentiation says d/dx (uv) = u dv/dx + v du/dx. We can apply the same rule in partial differentiation as well when there are two functions of the same variable.
Example:
β / βx (xy sin x) = y β / βx (x sin x)
= y [ x β / βx (sin x) + sin x β / βx (x) ]
= y [ x cos x + sin x]
Quotient Rule
The
quotient rule
of ordinary differentiation says d/dx (u/v) = [ v du/dx - u dv/dx ] / v
2
. As other rules, this rule can be applied for finding partial derivatives also.
Example:
β / βx ( xy / sin x)
= y β / βx (x / sin x)
= y [ ( sin x β / βx (x) - x β / βx (sin x) ) / sin
2
x ]
= y [ sin x - x cos x] / sin
2
x
Chain Rule of Partial Differentiation
The chain rule is used when we have to differentiate an
implicit function
. The chain rule of partial derivatives works a little differently when compared to ordinary derivatives. Sometimes, the rule involves both partial derivatives and ordinary derivatives. There are various forms of this rule and the application of one of them depends upon how each variable of the function is defined.
If y = f(x) is a function where x is again a function of two variables u and v (i.e., x = x (u, v)) then
βf/βu = βf/βx Β· βx/βu;
βf/βv = βf/βx Β· βx/βv
If z = f(x, y), where each of x and y are again functions of a variable t (i.e., x = x(t) and y = y(t)) then
df/dt = (βf/βx Β· dx/dt) + (βf/βy Β· dy/dt)
If z = f(x, y) is a function and each of x and y are again functions of two variables u and v (i.e., x = x(u, v) and y = y(u, v)) then
βf/βu = βf/βx Β· βx/βu + βf/βy Β· βy/βu;
βf/βv = βf/βx Β· βx/βv + βf/βy Β· βy/βv
Example:
If z = e
xy
, where x = uv and y = u + v then find the partial derivative βf/βu.
Solution:
By the chain rule of partial derivatives:
βf/βu = βf/βx Β· βx/βu + βf/βy Β· βy/βu
= β / βx (e
xy
) Β· β / βu (uv) + β / βy (e
xy
) Β· β / βu (u + v)
= (e
xy
Β· y) (v) + (e
xy
Β· x) (1)
= e
xy
(x + vy)
Other Rules of Partial Differentiation
If f(x, y) = a constant, then the following formula gives the relation between the ordinary derivative and the partial derivatives which follows from
implicit differentiation
.
dy/dx = -f
x
/f
y
.
For any two functions u(x, y) and v(x, y), the
determinant
|
β
u
/
β
x
β
u
/
β
y
β
v
/
β
x
β
v
/
β
y
|
is known as
Jacobian
of u and v.
The
Laplace equation
of partial derivatives is β
2
f / βx
2
+ β
2
f / βy
2
+ β
2
f / βz
2
= 0 where f(x, y, z) is a function in three variables. Any function f that satisfies the Laplace equation is known as the harmonic function.
Important Notes on Partial Derivatives:
While finding the partial derivative with respect to a variable, all the other variables should be considered as constants.
The order of taking derivatives doesn't matter in partial derivatives. i.e., β
2
f / βx βy = β
2
f / βy βx.
The rules of derivatives apply for partial differentiation as well.
Instead of using the limit definition, applying
derivative formulas
make the process of finding the partial derivatives easier.
β
Related Topics:
Partial Differential Equations
Partial Derivative Calculator
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# Partial Derivative
The partial derivative of a function (in two or more variables) is its derivative with respect to one of the variables keeping all the other variables as constants. The process of calculating partial derivative is as same as that of an ordinary derivative except we consider the other variables than the variable with respect to which we are differentiating as constants.
Let us learn more about how to calculate partial derivatives of different orders along with examples.
| | |
|---|---|
| 1\. | [What is Partial Derivative?](https://www.cuemath.com/calculus/partial-derivative/#a) |
| 2\. | [Partial Derivative Symbol](https://www.cuemath.com/calculus/partial-derivative/#b) |
| 3\. | [How to Calculate Partial Derivatives?](https://www.cuemath.com/calculus/partial-derivative/#c) |
| 4\. | [Partial Derivatives of Different Orders](https://www.cuemath.com/calculus/partial-derivative/#d) |
| 5\. | [Partial Differentiation Formulas](https://www.cuemath.com/calculus/partial-derivative/#e) |
| 6\. | [FAQs on Partial Derivative](https://www.cuemath.com/calculus/partial-derivative/#faqs) |
## What is Partial Derivative?
The **partial derivative** of a multivariable function, say z = f(x, y), is its [derivative](https://www.cuemath.com/calculus/differentiable/) with respect to one of the variables, x or y in this case, where the other variables are treated as constants. For example,
- for finding the partial derivative of f(x, y) with respect to x (which is represented by βf / βx), y is treated as constant and
- for finding the partial derivative of f(x, y) with respect to y (which is represented by βf / βy), x is treated as constant
Note that we are not considering all the variables as variables while doing partial differentiation (instead, we are considering only one variable as a variable at a time) and hence the name "partial". The limit definition of a partial derivative looks very similar to the [limit](https://www.cuemath.com/calculus/limits/) definition of the derivative. We can find the partial derivatives using the following limit formulas:
- βf / βx = lim h β 0 \[ f(x + h, y) - f(x, y) \] / h
- βf / βy = lim h β 0 \[ f(x, y + h) - f(x, y) \] / h

These formulas resemble the derivative definition using the first principle.
### Example of Partial Derivative
If f (x, y) = xy, then find the partial derivative βf / βx.
**Solution:**
βf / βx = lim h β 0 \[ f(x + h, y) - f(x, y) \] / h
= lim h β 0 \[ (x + h)y - xy \] / h
= lim h β 0 \[xy + hy - xy\] / h
= lim h β 0 \[hy\]/h
= lim h β 0 y
= y
Therefore, βf / βx = y.
## Partial Derivative Symbol
We know that the ordinary derivative of a function y = f(x) is denoted by one of the notations dy/dx, d/dx (y), d/dx (f(x)), f '(x), etc. For representing a partial derivative we use the symbol "β" instead of "d". We pronounce "β" to be "doh" but it has some other names like "partial", "del", "partial dee", "dee", "Jacobi's delta", etc. If z = f(x, y) is a function in two variables then
- βf / βx is the partial derivative of f with respect to x
- βf / βy is the partial derivative of f with respect to y
Just like how we have different [symbols](https://www.cuemath.com/numbers/math-symbols/) of ordinary derivatives, we have different notations for partial derivatives as well. For example, βf / βx can be written as fx, fx', Dxf, β / βx (f), βx f, β / βx \[f(x, y)\], βz / βx, etc.
## Calculate Partial Derivatives
We have already seen that the limit definitions are used to find the partial derivatives. But using the [limit formula](https://www.cuemath.com/limit-formula/) and computing the limit is not always easy. Thus, we have another method to calculate partial derivatives that follow right from its definition. In this method, if z = f(x, y) is the function, then we can compute the partial derivatives using the following steps:
- **Step 1**: Identify the variable with respect to which we have to find the partial derivative.
- **Step 2:** Except for the variable found in **Step 1**, treat all the other variables as constants.
- **Step 3:** Differentiate the function just using the rules of ordinary differentiation.
Wait! Read **Step 3** again. Yes, the rules of ordinary differentiation are as same as that of partial differentiation. In partial differentiation, just treating variables is different, that's it\!
**Example:** Let us solve the same above example (If f (x, y) = xy, then find the partial derivative βf / βx) using the above steps.
**Solution:**
We have to find βf / βx. It means, we have to find the partial derivative of f with respect to x. So we treat y as constant. Thus, we can write 'y' outside the derivative (as in ordinary differentiation, we have a rule that says d/dx (c y) = c dy/dx, where 'c' is a constant). Thus,
βf / βx = β / βx (xy)
= y β / βx (x)
= y (1) (Using [power rule](https://www.cuemath.com/calculus/power-rule/), d/dx (x) = 1)
= y
We have got the same answer as we got using the limit definition.
## Partial Derivatives of Different Orders
We have derivatives like first-order derivatives (like dy/dx), [second-order derivatives](https://www.cuemath.com/calculators/second-derivative-calculator/) (like d2y/dx2), etc in ordinary derivatives. Likewise, we have first-order, second-order, and higher-order derivatives in partial derivatives also.
### First Order Partial Derivatives
If z = f(x, y) is a function in two variables, then it can have two first-order partial derivatives, namely βf / βx and βf / βy.
**Example:** If z = x2 + y2, find all the first order partial derivatives.
**Solution:**
fx = βf / βx = β / βx (x2 + y2)
= β / βx (x2) + β / βx (y2)
= 2x + 0 (as y is a constant)
= 2x
fy = βf / βy = β / βy (x2 + y2)
= β / βy (x2) + β / βy (y2)
= 0 + 2y (as x is a constant)
= 2y
### Second Order Partial Derivatives
The second-order partial derivative is obtained by differentiating the function with respect to the indicated variables successively one after the other. If z = f(x, y) is a function in two variables, then it can have four second-order partial derivatives, namely β2f / βx2, β2f / βy2, β2f / βx βy and β2f / βy ββxβββ. To find them, we can first differentiate the function partially with the latter variable, and then partially differentiate the result with respect to the former variable. i.e.,
- fxx = β2f / βx2 = β / βx (βf / βx) = β / βx (fx)
- fyy = β2f / βy2 = β / βy (βf / βy) = β / βx (fy)
- fyx = β2f / βx βy = β / βx (βf / βy) = β / βx (fy)
- fxy = β2f / βy βx = β / βy (βf / βx) = β / βy (fx)
Observe the notations fyx and fxy. The order of variables in each subscript indicate the order of partial differentiation. For example, fyx means to partially differentiate with respect to y first and then with respect to x, and this is same as β2f / βx βy.
**Example:** If z = x2 + y2, find all the second order partial derivatives.
**Solution:**
In the above example, we have already found that fx = 2x and fy = 2y.
Now, fxx = β / βx (fx) = β / βx (2x) = 2
fyy = β / βy (fy) = β / βy (2y) = 2
fyx = β / βx (fy) = β / βx (2y) = 0
fxy = β / βy (fx) = β / βy (2x) = 0
Now that fyx = fxy. Thus, the order of partial differentiation doesn't matter.
## Partial Differentiation Formulas
The process of finding partial derivatives is known as Partial Differentiation. To find the first-order partial derivatives (as discussed earlier) of a function z = f(x, y) we use the following limit formulas:
- βf / βx = lim h β 0 \[ f(x + h, y) - f(x, y) \] / h
- βf / βy = lim h β 0 \[ f(x, y + h) - f(x, y) \] / h
But instead of using these formulas, just treating all the other variables than the variable with respect to which we are partially differentiating as constants would make the process of partial differentiation very easier. In this process, we just use the same [rules as ordinary differentiation](https://www.cuemath.com/calculus/derivative-rules/) and among them, the important rules are as follows:
### Power Rule
The power rule of differentiation says d/dx (xn) = n xn-1. The same rule can be applied in partial derivatives also.
**Example:** β / βx (x2y) = y β / βx (x2) = y (2x) = 2xy.
### Product Rule
The [product rule](https://www.cuemath.com/calculus/product-rule/) of ordinary differentiation says d/dx (uv) = u dv/dx + v du/dx. We can apply the same rule in partial differentiation as well when there are two functions of the same variable.
**Example:** β / βx (xy sin x) = y β / βx (x sin x)
= y \[ x β / βx (sin x) + sin x β / βx (x) \]
= y \[ x cos x + sin x\]
### Quotient Rule
The [quotient rule](https://www.cuemath.com/calculus/quotient-rule/) of ordinary differentiation says d/dx (u/v) = \[ v du/dx - u dv/dx \] / v2. As other rules, this rule can be applied for finding partial derivatives also.
**Example:** β / βx ( xy / sin x)
= y β / βx (x / sin x)
= y \[ ( sin x β / βx (x) - x β / βx (sin x) ) / sin2x \]
= y \[ sin x - x cos x\] / sin2x
### Chain Rule of Partial Differentiation
The chain rule is used when we have to differentiate an [implicit function](https://www.cuemath.com/algebra/implicit-function/). The chain rule of partial derivatives works a little differently when compared to ordinary derivatives. Sometimes, the rule involves both partial derivatives and ordinary derivatives. There are various forms of this rule and the application of one of them depends upon how each variable of the function is defined.
- If y = f(x) is a function where x is again a function of two variables u and v (i.e., x = x (u, v)) then
βf/βu = βf/βx Β· βx/βu;
βf/βv = βf/βx Β· βx/βv
- If z = f(x, y), where each of x and y are again functions of a variable t (i.e., x = x(t) and y = y(t)) then
df/dt = (βf/βx Β· dx/dt) + (βf/βy Β· dy/dt)
- If z = f(x, y) is a function and each of x and y are again functions of two variables u and v (i.e., x = x(u, v) and y = y(u, v)) then
βf/βu = βf/βx Β· βx/βu + βf/βy Β· βy/βu;
βf/βv = βf/βx Β· βx/βv + βf/βy Β· βy/βv
**Example:** If z = exy, where x = uv and y = u + v then find the partial derivative βf/βu.
**Solution:**
By the chain rule of partial derivatives:
βf/βu = βf/βx Β· βx/βu + βf/βy Β· βy/βu
= β / βx (exy) Β· β / βu (uv) + β / βy (exy) Β· β / βu (u + v)
= (exy Β· y) (v) + (exy Β· x) (1)
= exy (x + vy)
### Other Rules of Partial Differentiation
- If f(x, y) = a constant, then the following formula gives the relation between the ordinary derivative and the partial derivatives which follows from [implicit differentiation](https://www.cuemath.com/calculus/implicit-differentiation/).
dy/dx = -fx/fy.
- For any two functions u(x, y) and v(x, y), the [determinant](https://www.cuemath.com/algebra/determinant-of-matrix/)
β£β£β£βu/βxβu/βyβv/βxβv/βyβ£β£β£
\|
β
u
/
β
x
β
u
/
β
y
β
v
/
β
x
β
v
/
β
y
\|
is known as **Jacobian** of u and v.
- The **Laplace equation** of partial derivatives is β2f / βx2 + β2f / βy2 + β2f / βz2 = 0 where f(x, y, z) is a function in three variables. Any function f that satisfies the Laplace equation is known as the harmonic function.
**Important Notes on Partial Derivatives:**
- While finding the partial derivative with respect to a variable, all the other variables should be considered as constants.
- The order of taking derivatives doesn't matter in partial derivatives. i.e., β2f / βx βy = β2f / βy βx.
- The rules of derivatives apply for partial differentiation as well.
- Instead of using the limit definition, applying [derivative formulas](https://www.cuemath.com/derivative-formula/) make the process of finding the partial derivatives easier.
β **Related Topics:**
- [Partial Differential Equations](https://www.cuemath.com/calculus/partial-differential-equations/)
- [Partial Derivative Calculator](https://www.cuemath.com/calculators/partial-derivative-calculator/)
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## Partial Derivative Examples
1. **Example 1:** Find all the first order partial derivatives of the function f(x, y) = ax2 + 2hxy + by2.
**Solution:**
The first-order partial derivatives are:
fx = βf / βx = β / βx (ax2 + 2hxy + by2)
= β / βx (ax2) + β / βx (2hxy) + β / βx (by2)
= 2ax + 2hy (1) + 0 (as y is a constant)
= 2ax + 2hy
fx = βf / βx = β / βx (ax2 + 2hxy + by2)
= β / βy (ax2) + β / βy (2hxy) + β / βy (by2)
= 0 + 2hx (1) + 2by (as x is a constant)
= 2hx + 2by
**Answer:** fx = 2ax + 2hy and fy = 2hx + 2by.
2. **Example 2:** Find all the second-order partial derivatives of the function given in **Example 1.**
**Solution:**
We found in **Example 1** thatfx = 2ax + 2hy and fy = 2hx + 2by. Now, we will find the second-order partial derivatives.
fxx = β / βx (fx) = β / βx (2ax + 2hy) = 2a
fyy = β / βy (fy) = β / βy (2hx + 2by) = 2b
fyx = β / βx (fy) = β / βx (2hx + 2by) = 2h
fxy = β / βy (fx) = β / βy (2ax + 2hy) = 2h
**Answer:** We found that fxx = 2a, fyy = 2b, fyx = 2h, and fxy = 2h.
3. **Example 3:** Does the order of differentiation matter while finding the partial derivatives? Justify your answer using **Example 2.**
**Solution:**
From **Example 2**, fyx = 2h and fxy = 2h.
Thus, fxy = fxy.
Thus, the order of partial differentiation doesn't matter.
**Answer:** The order doesn't matter as fyx = fxy.
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## Practice Questions on Partial Differentiation
Check Answer \>
## FAQs on Partial Derivative
### How to Calculate Partial Derivatives?
We use **partial derivatives** when the function has more than one variable. If a function f is in terms of two variables x and y, then we can calculate the partial derivatives as follows.
- the partial derivative of f = βf/βx and y has to be treated as constant here.
- the partial derivative of f = βf/βy and x has to be treated as constant here.
### What is the Symbol of Partial Derivatives?
We use the symbol β to represent a partial derivative. For example, the partial derivative of a function f(x, y) with respect to x is written as βf/βx.
### What is the Difference Between Differentiation and Partial Differentiation?
We talk about the [derivative](https://www.cuemath.com/calculus/derivatives/) of a function if a function has only one variable in it. For example, the derivative of a function y = f(x) is denoted by df/dx. We talk about partial derivatives when a function z = f(x, y) has more than one variable. The partial derivative of f with respect to x is denoted by βf/βx and while finding this, we treat y as constant.
### What is the Formula Used to Find the Partial Derivative?
The partial derivatives of a function z = f(x, y) can be found using the limit formulas:
- βf / βx = lim h β 0 \[ f(x + h, y) - f(x, y) \] / h
- βf / βy = lim h β 0 \[ f(x, y + h) - f(x, y) \] / h
### What Does Partial Derivative Tell Us?
Partial derivative tells us to [differentiate](https://www.cuemath.com/calculus/differentiation/) a function partially. It means if we are differentiating partially with respect to one variable, then the remaining [variables](https://www.cuemath.com/algebra/variables-constants-and-expressions/) of the function must be treated as constants.
### What is the Chain Rule of Partial Derivatives?
The chain rule of partial derivative is mentioned below: If z = f(x, y) is a [function](https://www.cuemath.com/calculus/What-are-functions/) where x and y are functions of two variables u and v (i.e., x = x(u, v) and y = y(u, v)) then by the chain rule of partial derivatives,
- βf/βu = βf/βx Β· βx/βu + βf/βy Β· βy/βu
- βf/βv = βf/βx Β· βx/βv + βf/βy Β· βy/βv
### What is the Formula that Connects Normal Derivatives with Partial Derivatives?
When f(x, y) = c, where 'c' is a [constant](https://www.cuemath.com/algebra/constants/), then dy/dx = -fx/fy, where
- fx is the partial derivative of f with respect to x
- fy is the partial derivative of f with respect to y
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| Readable Markdown | The partial derivative of a function (in two or more variables) is its derivative with respect to one of the variables keeping all the other variables as constants. The process of calculating partial derivative is as same as that of an ordinary derivative except we consider the other variables than the variable with respect to which we are differentiating as constants.
Let us learn more about how to calculate partial derivatives of different orders along with examples.
| | |
|---|---|
| 1\. | [What is Partial Derivative?](https://www.cuemath.com/calculus/partial-derivative/#a) |
| 2\. | [Partial Derivative Symbol](https://www.cuemath.com/calculus/partial-derivative/#b) |
| 3\. | [How to Calculate Partial Derivatives?](https://www.cuemath.com/calculus/partial-derivative/#c) |
| 4\. | [Partial Derivatives of Different Orders](https://www.cuemath.com/calculus/partial-derivative/#d) |
| 5\. | [Partial Differentiation Formulas](https://www.cuemath.com/calculus/partial-derivative/#e) |
| 6\. | [FAQs on Partial Derivative](https://www.cuemath.com/calculus/partial-derivative/#faqs) |
The **partial derivative** of a multivariable function, say z = f(x, y), is its [derivative](https://www.cuemath.com/calculus/differentiable/) with respect to one of the variables, x or y in this case, where the other variables are treated as constants. For example,
- for finding the partial derivative of f(x, y) with respect to x (which is represented by βf / βx), y is treated as constant and
- for finding the partial derivative of f(x, y) with respect to y (which is represented by βf / βy), x is treated as constant
Note that we are not considering all the variables as variables while doing partial differentiation (instead, we are considering only one variable as a variable at a time) and hence the name "partial". The limit definition of a partial derivative looks very similar to the [limit](https://www.cuemath.com/calculus/limits/) definition of the derivative. We can find the partial derivatives using the following limit formulas:
- βf / βx = lim h β 0 \[ f(x + h, y) - f(x, y) \] / h
- βf / βy = lim h β 0 \[ f(x, y + h) - f(x, y) \] / h
These formulas resemble the derivative definition using the first principle.
### Example of Partial Derivative
If f (x, y) = xy, then find the partial derivative βf / βx.
**Solution:**
βf / βx = lim h β 0 \[ f(x + h, y) - f(x, y) \] / h
= lim h β 0 \[ (x + h)y - xy \] / h
= lim h β 0 \[xy + hy - xy\] / h
= lim h β 0 \[hy\]/h
= lim h β 0 y
= y
Therefore, βf / βx = y.
We know that the ordinary derivative of a function y = f(x) is denoted by one of the notations dy/dx, d/dx (y), d/dx (f(x)), f '(x), etc. For representing a partial derivative we use the symbol "β" instead of "d". We pronounce "β" to be "doh" but it has some other names like "partial", "del", "partial dee", "dee", "Jacobi's delta", etc. If z = f(x, y) is a function in two variables then
- βf / βx is the partial derivative of f with respect to x
- βf / βy is the partial derivative of f with respect to y
Just like how we have different [symbols](https://www.cuemath.com/numbers/math-symbols/) of ordinary derivatives, we have different notations for partial derivatives as well. For example, βf / βx can be written as fx, fx', Dxf, β / βx (f), βx f, β / βx \[f(x, y)\], βz / βx, etc.
We have already seen that the limit definitions are used to find the partial derivatives. But using the [limit formula](https://www.cuemath.com/limit-formula/) and computing the limit is not always easy. Thus, we have another method to calculate partial derivatives that follow right from its definition. In this method, if z = f(x, y) is the function, then we can compute the partial derivatives using the following steps:
- **Step 1**: Identify the variable with respect to which we have to find the partial derivative.
- **Step 2:** Except for the variable found in **Step 1**, treat all the other variables as constants.
- **Step 3:** Differentiate the function just using the rules of ordinary differentiation.
Wait! Read **Step 3** again. Yes, the rules of ordinary differentiation are as same as that of partial differentiation. In partial differentiation, just treating variables is different, that's it\!
**Example:** Let us solve the same above example (If f (x, y) = xy, then find the partial derivative βf / βx) using the above steps.
**Solution:**
We have to find βf / βx. It means, we have to find the partial derivative of f with respect to x. So we treat y as constant. Thus, we can write 'y' outside the derivative (as in ordinary differentiation, we have a rule that says d/dx (c y) = c dy/dx, where 'c' is a constant). Thus,
βf / βx = β / βx (xy)
= y β / βx (x)
= y (1) (Using [power rule](https://www.cuemath.com/calculus/power-rule/), d/dx (x) = 1)
= y
We have got the same answer as we got using the limit definition.
We have derivatives like first-order derivatives (like dy/dx), [second-order derivatives](https://www.cuemath.com/calculators/second-derivative-calculator/) (like d2y/dx2), etc in ordinary derivatives. Likewise, we have first-order, second-order, and higher-order derivatives in partial derivatives also.
### First Order Partial Derivatives
If z = f(x, y) is a function in two variables, then it can have two first-order partial derivatives, namely βf / βx and βf / βy.
**Example:** If z = x2 + y2, find all the first order partial derivatives.
**Solution:**
fx = βf / βx = β / βx (x2 + y2)
= β / βx (x2) + β / βx (y2)
= 2x + 0 (as y is a constant)
= 2x
fy = βf / βy = β / βy (x2 + y2)
= β / βy (x2) + β / βy (y2)
= 0 + 2y (as x is a constant)
= 2y
### Second Order Partial Derivatives
The second-order partial derivative is obtained by differentiating the function with respect to the indicated variables successively one after the other. If z = f(x, y) is a function in two variables, then it can have four second-order partial derivatives, namely β2f / βx2, β2f / βy2, β2f / βx βy and β2f / βy ββxβββ. To find them, we can first differentiate the function partially with the latter variable, and then partially differentiate the result with respect to the former variable. i.e.,
- fxx = β2f / βx2 = β / βx (βf / βx) = β / βx (fx)
- fyy = β2f / βy2 = β / βy (βf / βy) = β / βx (fy)
- fyx = β2f / βx βy = β / βx (βf / βy) = β / βx (fy)
- fxy = β2f / βy βx = β / βy (βf / βx) = β / βy (fx)
Observe the notations fyx and fxy. The order of variables in each subscript indicate the order of partial differentiation. For example, fyx means to partially differentiate with respect to y first and then with respect to x, and this is same as β2f / βx βy.
**Example:** If z = x2 + y2, find all the second order partial derivatives.
**Solution:**
In the above example, we have already found that fx = 2x and fy = 2y.
Now, fxx = β / βx (fx) = β / βx (2x) = 2
fyy = β / βy (fy) = β / βy (2y) = 2
fyx = β / βx (fy) = β / βx (2y) = 0
fxy = β / βy (fx) = β / βy (2x) = 0
Now that fyx = fxy. Thus, the order of partial differentiation doesn't matter.
The process of finding partial derivatives is known as Partial Differentiation. To find the first-order partial derivatives (as discussed earlier) of a function z = f(x, y) we use the following limit formulas:
- βf / βx = lim h β 0 \[ f(x + h, y) - f(x, y) \] / h
- βf / βy = lim h β 0 \[ f(x, y + h) - f(x, y) \] / h
But instead of using these formulas, just treating all the other variables than the variable with respect to which we are partially differentiating as constants would make the process of partial differentiation very easier. In this process, we just use the same [rules as ordinary differentiation](https://www.cuemath.com/calculus/derivative-rules/) and among them, the important rules are as follows:
### Power Rule
The power rule of differentiation says d/dx (xn) = n xn-1. The same rule can be applied in partial derivatives also.
**Example:** β / βx (x2y) = y β / βx (x2) = y (2x) = 2xy.
### Product Rule
The [product rule](https://www.cuemath.com/calculus/product-rule/) of ordinary differentiation says d/dx (uv) = u dv/dx + v du/dx. We can apply the same rule in partial differentiation as well when there are two functions of the same variable.
**Example:** β / βx (xy sin x) = y β / βx (x sin x)
= y \[ x β / βx (sin x) + sin x β / βx (x) \]
= y \[ x cos x + sin x\]
### Quotient Rule
The [quotient rule](https://www.cuemath.com/calculus/quotient-rule/) of ordinary differentiation says d/dx (u/v) = \[ v du/dx - u dv/dx \] / v2. As other rules, this rule can be applied for finding partial derivatives also.
**Example:** β / βx ( xy / sin x)
= y β / βx (x / sin x)
= y \[ ( sin x β / βx (x) - x β / βx (sin x) ) / sin2x \]
= y \[ sin x - x cos x\] / sin2x
### Chain Rule of Partial Differentiation
The chain rule is used when we have to differentiate an [implicit function](https://www.cuemath.com/algebra/implicit-function/). The chain rule of partial derivatives works a little differently when compared to ordinary derivatives. Sometimes, the rule involves both partial derivatives and ordinary derivatives. There are various forms of this rule and the application of one of them depends upon how each variable of the function is defined.
- If y = f(x) is a function where x is again a function of two variables u and v (i.e., x = x (u, v)) then
βf/βu = βf/βx Β· βx/βu;
βf/βv = βf/βx Β· βx/βv
- If z = f(x, y), where each of x and y are again functions of a variable t (i.e., x = x(t) and y = y(t)) then
df/dt = (βf/βx Β· dx/dt) + (βf/βy Β· dy/dt)
- If z = f(x, y) is a function and each of x and y are again functions of two variables u and v (i.e., x = x(u, v) and y = y(u, v)) then
βf/βu = βf/βx Β· βx/βu + βf/βy Β· βy/βu;
βf/βv = βf/βx Β· βx/βv + βf/βy Β· βy/βv
**Example:** If z = exy, where x = uv and y = u + v then find the partial derivative βf/βu.
**Solution:**
By the chain rule of partial derivatives:
βf/βu = βf/βx Β· βx/βu + βf/βy Β· βy/βu
= β / βx (exy) Β· β / βu (uv) + β / βy (exy) Β· β / βu (u + v)
= (exy Β· y) (v) + (exy Β· x) (1)
= exy (x + vy)
### Other Rules of Partial Differentiation
- If f(x, y) = a constant, then the following formula gives the relation between the ordinary derivative and the partial derivatives which follows from [implicit differentiation](https://www.cuemath.com/calculus/implicit-differentiation/).
dy/dx = -fx/fy.
- For any two functions u(x, y) and v(x, y), the [determinant](https://www.cuemath.com/algebra/determinant-of-matrix/)
\|
β
u
/
β
x
β
u
/
β
y
β
v
/
β
x
β
v
/
β
y
\|
is known as **Jacobian** of u and v.
- The **Laplace equation** of partial derivatives is β2f / βx2 + β2f / βy2 + β2f / βz2 = 0 where f(x, y, z) is a function in three variables. Any function f that satisfies the Laplace equation is known as the harmonic function.
**Important Notes on Partial Derivatives:**
- While finding the partial derivative with respect to a variable, all the other variables should be considered as constants.
- The order of taking derivatives doesn't matter in partial derivatives. i.e., β2f / βx βy = β2f / βy βx.
- The rules of derivatives apply for partial differentiation as well.
- Instead of using the limit definition, applying [derivative formulas](https://www.cuemath.com/derivative-formula/) make the process of finding the partial derivatives easier.
β **Related Topics:**
- [Partial Differential Equations](https://www.cuemath.com/calculus/partial-differential-equations/)
- [Partial Derivative Calculator](https://www.cuemath.com/calculators/partial-derivative-calculator/)
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