Answer
$\sin(x) = \cos(90^\circ - x)$, or in radians $\sin(x) = \cos\left(\frac{\pi}{2} - x\right)$.
Explanation
A Digital SAT staple: if $\sin(a) = \cos(b)$ for acute angles $a$ and $b$ in a right triangle, then $a + b = 90^\circ$ (or $\frac{\pi}{2}$).