$k=$ | $\frac{\pi}{2}+2\pi k=$ | $\frac{3\pi}{2}+2\pi k=$ |
$-1$ | $-\frac{3\pi}{2}=-\frac{9\pi}{6}$ | $-\frac{\pi}{2}=-\frac{3\pi}{6}$ |
$0$ | $\frac{\pi}{2}=\frac{3\pi}{6}$ | $\frac{3\pi}{2}=\frac{9\pi}{6}$ |
$1$ | $\frac{5\pi}{2}=\frac{15\pi}{6}$ | $\frac{7\pi}{2}=\frac{21\pi}{6}$ |