cos(二分之3π-α)= -sinα证明

问题描述:

cos(二分之3π-α)= -sinα证明

cos(3/2π-α)= cos[(π+(π/2-α)]=
α为锐角时π+(π/2-α)在第三象限
cos(3/2π-α)= cos[(π+(π/2-α)]= - cos(π/2-α)= -sinα