CPM Homework Help : PC3 Problem 11-70

Evaluate each of the following trigonometric expressions. Give exact answers.

$\cos( \frac { x } { 2 } )$ if $\cos\left(x\right)=\frac{1}{6},\ \pi \leq x \leq 2\pi$
Step (b):
$\pm\sqrt{\frac{1+\cos(x)}{2}}=\pm\sqrt{\frac{1+1/6}{2}}$ if $\cos(x) = \frac { 1 } { 6 }$ , $π ≤ x < 2π$