Cos 135°cos 105° помагите

Решите задачу:

$\displaystyle cos135^{\circ}cos105^{\circ}= \frac{1}{2}(cos(135+105)+cos(135-105))=\frac{1}{2}(cos240^{\circ}+$
$\displaystyle +cos30^{\circ})= \frac{1}{2}(cos(270^{\circ}-30^{\circ})+cos30^{\circ})= \frac{1}{2}(-sin30^{\circ}+ \frac{ \sqrt3}{2})=$ $\displaystyle = \frac{1}{2}(-\frac{1}{2}+ \frac{ \sqrt3}{2})= \boxed{\frac{ \sqrt 3-1}{4}}$