已知：sin(x-3p/4)cos(x-p/4)=-1/4,则cos4x的等于____.
问题描述：
已知：sin(x-3p/4)cos(x-p/4)=-1/4,则cos4x的等于____.
答
sin(x-3p/4)cos(x-p/4)=-1/4sin(x-3p/4)sin[p/2-(x-p/4)]=-1/4sin(x-3p/4)sin(3p/4-x)=-1/4sin(3p/4-x)sin(x-3p/4)=1/4[sin(3p/4-x)]^2=1/4[sin(p-p/4-x)]^2=1/4[sin(x+p/4)]^2=1/4cos4x=-cos(4x+p)=-cos[4*(x+p/4)]...