根据三角函数积化和差公式： sinα·sinβ=-(1/2)[cos(α+β)-cos(α-β)] sin(y-x)*sinx =-(1/2)[cos(y-x+x)-cos(y-x-x)] =-(1/2)[cosy-cos(y-2x)] =-1/2cosy+1/2cos(y-2x)
