The function is , .

The Mean value theorem :

Let be a function that satisfies the following three hypotheses.

1. is continuous on .

2. is differentiable on .

Then there is a number  in such that

.

The function is , .

It is a polynomial function hence, it is continuous and differentiable.

Differentiate on each side with respect to .

.

From the mean value theorem,

Therefore .

.

.