Question

cos(a-П/2)-sin(a-П/2)

Answer 1

$displaystyle cos(a-frac{pi }{2})-sin(a-frac{pi }{2})=cos(-(frac{pi }{2}-a))-sin(-(frac{pi }{2}-a))=\\=cos(frac{pi}{2}-a)+sin(frac{pi}{2}-a)=sina+cosa$