sin2x=-cos(2x+π/2)= -1+2sin²(x+π/4)
sin(x+π/4)=√（sin2x+1）/2
f(x)=cos2x/(√（sin2x+1）/2)=4/3
sin2x=-1 或 1/9
随便算的 不晓得对不对哈