Question

sin%28%20%5Calpha%20%29%20" id="TexFormula1" title=" \cos( \alpha + \frac{\pi}{2} ) = - \sin( \alpha ) " alt=" \cos( \alpha + \frac{\pi}{2} ) = - \sin( \alpha ) " align="absmiddle" class="latex-formula">
why is it equal​

Answer 1

$\cos (\alpha +\frac{\pi}{2})= \cos(\frac{\pi}{2})\cos (\alpha)- \sin(\frac{\pi}{2})\sin(\alpha)\\=0-\sin(\alpha)\\=-\sin (\alpha)$