已知sin(x-3π/4)cos(x-π/4)=-1/4,则cosx等于
问题描述：
已知sin(x-3π/4)cos(x-π/4)=-1/4,则cosx等于
答
sin(x-3π/4)=sin[π-(π/4-x)]=sin(π/4-x)=-sin(x-π/4)sin(x-3π/4)cos(x-π/4)=-sin(x-π/4)cos(x-π/4)=-sin(2x-π/2)/2=-1/4cos(2x)=1/22x=60°x=30°cosx=cos30等于根号3/2