y=sin(x-π/4)cos(x+π/4)=sin(x-π/4)sin(π/4-x)=-sin²(x-π/4)=(cos2x-1)/2
y=sin(x-π/4)cos(x+π/4),x属于[0,π/4]的反函数
f^-1(x)=[arccos(2x+1)]/2,x属于[-1/2,1/2]