The function is .

Mean value theorem :

Let  be a function that satisfies the following hypotheses :

1.  is continuous on .

2.  is differentiable on .

Then there is a number  in  such that .

The function is .

The function is  continuous on the interval .

Differentiate  with respect to .

.

The function is differentiable on the interval .

The mean value theorem satisfies the hypothesis.

Then 

From the mean value theorem :

.

Substitute  in .

Substitute  in .

and .

 and .