[tex]\begin{gathered} \text{ Recall that cos x = sin (}\frac{\pi}{2}\text{ - x )} \\ \text{Therefore,} \\ \cos \text{ }\frac{\pi}{10}\text{ = sin (}\frac{\pi}{2}\text{ - }\frac{\pi}{10}) \\ \\ =\sin (\frac{5\pi-\pi}{10}) \\ =\text{ sin (}\frac{4\pi}{10}) \\ =\sin \text{ }\frac{2\pi}{5} \end{gathered}[/tex]
