The general solution of cos2θ=0\cos 2\theta = 0cos2θ=0 is:
θ=π4+nπ2\theta = \dfrac{\pi}{4} + \dfrac{n\pi}{2}θ=4π+2nπ
θ=π2+nπ\theta = \dfrac{\pi}{2} + n\piθ=2π+nπ
θ=nπ\theta = n\piθ=nπ
θ=π4+nπ\theta = \dfrac{\pi}{4} + n\piθ=4π+nπ