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Mathematics LibreTexts

1.9: Partial Derivatives

Definition of a Partial Derivative

Let \(f(x,y)\) be a function of two variables. Then we define the partial derivatives as:

Definition: Partial Derivative

\[ f_x = \dfrac{\partial f}{\partial x} = \lim_{h\to{0}} \dfrac{f(x+h,y)-f(x,y)}{h}  \]

\[ f_y = \dfrac{\partial f}{\partial y} = \lim_{h\to{0}} \dfrac{f(x,y+h)-f(x,y)}{h}  \]

if these limits exist.

Algebraically, we can think of the partial derivative of a function with respect to \(x\) as the derivative of the function with \(y\) held constant. Geometrically, the derivative with respect to \(x\) at a point \(P\) represents the slope of the curve that passes through \(P\) whose projection onto the \(xy\) plane is a horizontal line (if you travel due East, how steep are you climbing?)

Example 1


\[  f(x,y) = 2x + 3y \]


\[\begin{align} \dfrac{\partial f}{ \partial } &= \lim_{h\to{0}}\dfrac{(2(x+h)+3y) - (2x+3y)}{h} \\ &= \lim_{h\to{0}} \dfrac{2x+2h+3y-2x-3y}{h} \\ &= \lim_{h\to{0}} \dfrac{2h}{h} =2 . \end{align}\]

We also use the notation \(f_x\)  and \(f_y\) for the partial derivatives with respect to \(x\) and \(y\) respectively.

Find \(f_y\) for the function from the example above.

Finding Partial Derivatives the Easy Way

Since a partial derivative with respect to \(x\) is a derivative with the rest of the variables held constant, we can find the partial derivative by taking the regular derivative considering the rest of the variables as constants.

Example 2


\[ f(x,y)  =  3xy^2 - 2x^2y \]


\[  f_x  =  3y^2 - 4xy\]


\[ f_y  =  6xy - 2x^2. \]


 Find both partial derivatives for

  1. \(f(x,y) = xy \sin x \)

  2. \( f(x,y) =  \dfrac{ x + y}{ x - y}\).

Higher Order Partials

Just as with function of one variable, we can define second derivatives for functions of two variables. For functions of two variables, we have four types:  \( f_{xx}\), \(f_{xy}\), \(f_{yx}\), and \(f_{yy}\).

Example 3


\[f(x,y) = ye^x  \]


\[f_x = ye^x  \]


\[f_y=e^x.  \]

Now taking the partials of each of these we get:

\[f_{xx}=ye^x \;\;\; f_{xy}=e^x \;\;\; \text{and} \;\;\; f_{yy}=0 . \]

Notice that   

\[ f_{x,y} = f_{yx}.\]


Let \(f(x,y)\) be a function with continuous second order derivatives, then 

\[f_{xy} = f_{yx}.  \]

Functions of More Than Two Variables

Suppose that

\[  f(x,y,z)  =  xy - 2yz  \]

is a function of three variables, then we can define the partial derivatives in much the same way as we defined the partial derivatives for three variables.  

We have

\[f_x=y \;\;\; f_y=x-2z \;\;\; \text{and} \;\;\; f_z=-2y . \]

Example 4: The Heat Equation

Suppose that a building has a door open during a snowy day.  It can be shown that the equation

\[   H_t  =  c^2H_{xx} \]

models this situation where \(H\) is the heat of the room at the point \(x\) feet away from the door at time \(t\).  Show that

\[  H = e^{-t} \cos(\frac{x}{c}) \]

satisfies this differential equation.


We have

\[H_t = -e^{-t} \cos (\dfrac{x}{c})  \]

\[H_x =   -\dfrac{1}{c} e^{-t} \sin(\frac{x}{c})\]

\[H_{xx} = -\dfrac{1}{c^2}  e^{-t} \cos(\dfrac{x}{c}) .\]

So that

\[c^2 H_{xx}= -e^{-t} \cos (\dfrac{x}{c}) . \]

And the result follows.


  • Integrated by Justin Marshall.