若x=π/12,则cos(2x-π)*cos[2(x-π)]=?
问题描述：
若x=π/12,则cos(2x-π)*cos[2(x-π)]=?
答
原式=cos(-5π/6)*cos(-11π/6) =cos(5π/6)*cos(11π/6) =-cos²(5π/6) =[cos(5π/3)-1]/2 =[-cos(2π/3)-1]/2 =(1/2-1)/2 =-1/4