What is the value of $\sin {15}^{\circ }+\cos {15}^{\circ }$

$$\begin{gathered} \sin {15}^{\circ }+\cos {15}^{\circ } \\ =\sqrt{2}\left[\frac{1}{\sqrt{2}}\sin {15}^{\circ }+\frac{1}{\sqrt{2}}\cos {15}^{\circ }\right] \\ =\sqrt{2}\left[cos 45° \sin {15}^{\circ }+\sin 45° \cos {15}^{\circ }\right] \\ =\sqrt{2} \sin (45°+15°) \\ =\sqrt{2}\times \frac{\sqrt{3}}{2} \\ =\frac{\sqrt{3}}{\sqrt{2}} \end{gathered}$$