Answer
$a$
Work Step by Step
$\cos{(165)} = \cos{(120+45)} = \cos{(120)}\cos{(45)} - \sin{(120)} \sin{(45)}$
$\cos{(165)} = \dfrac{-1}{2} \times \dfrac{\sqrt{2}}{2} - \dfrac{\sqrt{3}}{2} \times \dfrac{\sqrt{2}}{2}$
$\cos{(165)} = \dfrac{-\sqrt{6}-\sqrt{2}}{4}$
