\cos( \alpha + (\pi)/(2) ) = - \sin( \alpha ) why is it equal​

Question

`\cos( \alpha + (\pi)/(2) ) = - \sin( \alpha )`
why is it equal​

Answer 1

`</p><p>\cos (\alpha +(\pi)/(2))= \cos((\pi)/(2))\cos (\alpha)- \sin((\pi)/(2))\sin(\alpha)\\</p><p>=0-\sin(\alpha)\\</p><p>=-\sin (\alpha)`