## Trigonometry (10th Edition)

$\theta$ = 15$^{\circ}$
$\cos$(2$\theta$ + 50$^{\circ}$) = $\sin$(2$\theta$ - 20$^{\circ}$) $\cos$ and $\sin$ are cofunctions if the sum of the angles is 90$^{\circ}$ Therefore: (2$\theta$ + 50$^{\circ}$) + (2$\theta$ - 20$^{\circ}$) = 90$^{\circ}$ 4$\theta$ + 30$^{\circ}$ = 90$^{\circ}$ 4$\theta$ = 60$^{\circ}$ $\theta$ = 15$^{\circ}$