Explanation:
(2)
cos(2θ) tan θ + sin θ = 0
tan θ (cos(2θ) + cos θ) = 0
tan θ = 0
θ = kπ
cos(2θ) + cos θ = 0
cos(2θ) = -cos θ
cos(2θ) = cos(π − θ)
2θ = π − θ + 2kπ
3θ = (2k + 1) π
θ = (2k + 1) π / 3
Therefore, θ = kπ or (2k + 1) π / 3.
(3)
2 cos² θ − 2 cos²(2θ)
Power reduction formula:
1 + cos(2θ) − (1 + cos(4θ))
cos(2θ) − cos(4θ)