(1-cos40)/2+(1+cos160)/2
=1/2-1/2cos40+1/2+1/2cos160
=1-1/2cos40+1/2cos(180-20)
=1-1/2cos40-1/2cos201/2-1/2cos40+1/2+1/2cos160这步怎么来的(a+b)/c =a/c+b/c