Rolles Theorem :

Let  be a function that satisfies the following three hypotheses.

1.   is continuous on .

2.   is differentiable on .

3. .

Then there is a number  in  such that .

The function is , over the interval .

Substitute  in the function.

Substitute  in the function.

.

Differentiate on each side .

Denominator should be zero.

So the function is not differentiable over 

; but  f is not differentiable on .