(sinx+siny)^2+(cosx+cosy)^2为什么等于1+1+2(sinxsiny+cosxcosy)=2+2cos(x-y)

(sinx+siny)^2+(cosx+cosy)^2= sinx^2 + siny^2 + 2sinxsiny + cosx^2 + cosy^2 + 2cosxcosy=（sinx^2 + cosx^2）+（siny^2 + cosy^2）+ 2sinxsiny + 2cosxcosy= 1 + 1 + 2 (sinxsiny + cosxcosy）再根据 余弦差交公...