Chapter 2 - Acute Angles and Right Triangles - Section 2.1 Trigonometric Functions of Acute Angles - 2.1 Exercises - Page 52: 28

$\cos$($\theta$ + 20$^{\circ}$) = $\sin$(70$^{\circ}$ - $\theta$ )

According to the Cofunction Identities $\cos$$A$ = $\sin$(90$^{\circ}$ - $A$) $\cos$($\theta$ + 20$^{\circ}$) Therefore: $\cos$($\theta$ + 20$^{\circ}$) = $\sin$(90$^{\circ}$ - 20$^{\circ}$ - $\theta$ ) $\cos$($\theta$ + 20$^{\circ}$) = $\sin$(70$^{\circ}$ - $\theta$ )