Precalculus (6th Edition) Blitzer

$\frac{5}{8}$
$sin^{2}t$ + $cos^{2}t$ = 1 sin t = $\frac{\sqrt {39}}{8}$ $(\frac{\sqrt {39}}{8})^{2}$ + $cos^{2}t$ = 1 $\frac{39}{64}$ + $cos^{2}t$ = 1 $cos^{2}t$ = 1 -$\frac{39}{64}$ $cos^{2}t$ = $\frac{25}{64}$ cos t = $\sqrt \frac{25}{64}$ = $\frac{5}{8}$