Question

三角函数的和差化积公式：COSX+COSY=?COSX-COSY=?

Answer 1

sinx+siny=2sin[(x+y)/2]cos[(x-y)/2]

sinx-siny=2cos[(x+y)/2]sin[(x-y)/2]

cosx+cosy=2cos[(x+y)/2]cos[(x-y)/2]

cosx-cosy=-2sin[(x+y)/2]sin[(x-y)/2]