令a=x+y b=x-y 则x=(a+b)/2,y=(a-b)/2 sina+sinb=sin(x+y)+sin(x-y) =sinxcosy+sinycosx+sinxcosy-sinycosx =2sinxcosy 将 x=(a+b)/2,y=(a-b)/2代入 得sina+sinb=2[sin(a+b)/2][cos(a-b)/2]