#### To determine

**To state:** The graph of the function *f* is not differentiable at which the numbers.

#### Answer

The function *f* is not differentiable at 1 and 5.

#### Explanation

**Note:**

The function *f* is not differentiable at the point *a,* then it must be satisfied any of the following conditions.

(i) The function *f* is discontinuous at the point *a.*

(ii) The function *f* has a corner point at the point *a.*

(iii) The function *f* has a vertical tangent at the point *a.*

From the graph of *f*, it is observed that f(1) is undefined. Therefore, the graph of *f* has a discontinuity at x=1. Thus it is concluded that, *f* is not differentiable at the point x=1.

The graph of *f* has a vertical tangent at the point x=5. Therefore, by note-(iii), *f* is not differentiable at the point x=5.