Question

Find the intervals on which f is increasing and decreasing. f(x)=-5cos^2x on [-pi,pi]

Answer 1

Attached is screenshot of problem and correct answer. I understand how intervals work, but the trig values just confuse me in this.

Answer 2

After solving, i get cos x=0 and sin x=0. On the interval [-pi,pi] that means for cosx=0 x=pi/2 and -pi/2. For sin x=0 x=0,pi. Don't know where to go from there.