sinαsinβ=-1/2[cos(α+β)-cos(α-β)] =-1/2[（cosαcosβ-sinαsinβ)-(cosαcosβ+sinαsinβ)] =-1/2[-2sinαsinβ] 其他的也是相同的证明方法：cosαcosβ= 1/2[cos(α+β)+cos(α-β)] sinαcosβ= 1/2[sin(...