Answer
$\pm \frac{\sqrt 3}{2}$
Work Step by Step
Using double angle formula, we can write
$\cos 2x=2\cos^{2}x-1=\frac{1}{2}$
$\implies 2\cos^{2}x=\frac{1}{2}+1=\frac{3}{2}$
$\implies \cos^{2}x=\frac{3}{2}\times\frac{1}{2}=\frac{3}{4}$
$\implies \cos x=\pm\sqrt {\frac{3}{4}}=\pm\frac{\sqrt 3}{2}$
