# How do you express cos((27pi)/8) as a trig function of an angle in Quadrant I?

$- \cos \left(\frac{3 \pi}{8}\right)$
$\cos \left(\frac{27 \pi}{8}\right) = \cos \left(\frac{11 \pi}{8} + 2 \pi\right) = \cos \left(\frac{11 \pi}{8}\right) =$
$= \cos \left(\frac{3 \pi}{8} + \pi\right) = - \cos \left(\frac{3 \pi}{8}\right)$