sina+sinb=sin[(a+b)/2+(a-b)/2]+sin[(a+b)/2-(a-b)/2]=sin(a+b)/2cos(a-b)/2+cos(a+b)/2sin(a-b)/2+sin(a+b)/2cos(a-b)/2-cos(a+b)/2sin(a-b)/2=2sin(a+b)/2cos(a-b)/2结果不明白怎么