为什么 (cos20°*cos40°)=1/2(cos20°-cos60°)

问题描述:

为什么 (cos20°*cos40°)=1/2(cos20°-cos60°)

积化和差
cosαcosβ=[cos(α+β)+cos(α-β)]/2
cos20°*cos40°
=[cos(20°+cos40°)+cos(20°-40°)]/2
=[cos60°+cos(-20°)]/2
=(cos60°+cos20°)/2