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

问题描述:

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

x=x/2+x/2=(x/2+y/2)+(x/2-y/2)
y=(x/2+y/2)-(x/2-y/2)
所以左边=sin(x/2+y/2)cos(x/2-y/2)+cos(x/2+y/2)sin(x/2-y/2)-sin(x/2+y/2)cos(x/2-y/2)+cos(x/2+y/2)sin(x/2-y/2)
=2cos(x/2+y/2)sin(x/2-y/2)