Product rule of derivative is . In words we can read as derivative of product of two functions is equal to derivative of first function second function as it is plus first function as it is derivative of second function. This product rule can be proving using first principle or derivative by definition.

Consider a function of the form .

First we take the increment or small change in the function.

Putting the value of function in the above equation, we get

Subtracting and adding on the right hand side, we have

Dividing both sides by , we get

Taking limit of both sides as , we have

__NOTE__**:** If we extended product of three function, then ** **

__Example__**:** Find the derivative of

We have the given function as

Differentiation with respect to variable , we get

Now using the formula derivative of a square root, we have