[tex]\displaystyle \cos 120\textdegree=\cos 2*60\textdegree =\cos ^260\textdegree - \sin ^260\textdegree=(\frac{1}{2})^2-(\frac{\sqrt3}{2})^2=\frac{1}{4}-\frac{3}{4}=\frac{-2}{4}=-\frac{1}{2} \\ \\ \cos 120\textdegree + \sin ^260\textdegree=-\frac{1}{2}+(\frac{\sqrt3}{2})^2=-\frac{1}{2}+\frac{3}{4}=\frac{-2+3}{4}=\frac{1}{4}[/tex]
