Trigonometry (10th Edition)

Between 0$^{\circ}$ and 360$^{\circ}$ $\cos$ $\theta$ = -$\sin$ $\theta$ is true for 135$^{\circ}$ and 315$^{\circ}$
$\theta$ = 135$^{\circ}$ $\cos$ 135$^{\circ}$ = -$\frac{\sqrt2}{2}$ $\theta$ = 315$^{\circ}$ $\sin$ 315$^{\circ}$ = $\frac{\sqrt2}{2}$ Therefore: -$\sin$ 315$^{\circ}$ = -$\frac{\sqrt2}{2}$ Therefore, $\cos$ 135$^{\circ}$ = -$\sin$ 315$^{\circ}$ Therefore: Between 0$^{\circ}$ and 360$^{\circ}$ $\cos$ $\theta$ = -$\sin$ $\theta$ is true for 135$^{\circ}$ and 315$^{\circ}$