Value of cos 15⁰

$$\begin{gathered} \cos {15}^0=\cos ({45}^0-{30}^0) \\ =\cos {45}^0\cos {30}^0+\sin {45}^0 \sin {30}^0 \\ =\frac{1}{\sqrt{2}}\frac{\sqrt{3}}{2}+\frac{1}{\sqrt{2}}\frac{1}{2} \\ =\frac{\sqrt{3+1}}{2\sqrt{2}} \end{gathered}$$