怎么用2角和与差的余弦公式推出2角和与差的正弦公式

网友回答

cos(x+y)=cosx cosy-sinx siny
sin(x+y)=cos(x+y-π/2))=cosx cos(y-π/2)-sinx sin(y-π/2)=cosx siny+sinx cosy
两角差的余弦就是把y用-y代替即可.