The trigonometric equation is .

Consider .

Substitute the double angle formula .

Apply zero product rule.

and

First part

The general solution of is , where is any integer.

solution is

Substitute in , hence

Substitute in , hence

Substitute in , hence

Hence, .

Therefore,

Therefore,  the solutions in the interval are .

.

Second part

No real solution for.

Therefore, the solution set is .

The solution for is .