The function is .

is differentiable at all values of  because it is a trigonometric function.

Critical number :

A critical number of a function is a number in the domain of such that either or does not exist.

Therefore, solutions of are the critical numbers.

Differentiate on each side with respect to .

Equate to zero.

and

and .

The general solution of is , where is an integer.

The general solution of is , where is an integer.

Consider .

.

The general solution is .

Consider

The general solution is .

The critical numbers are all numbers of the form ( is an integer).

The critical numbers are all numbers of the form ( is an integer).