y=[(sinx)^2+(cosx)^2]^2-2(sinx)^2+(cosx)^2
=1^2-1/2*(2sinxcosx)^2
=-1/2*(sin2x)^2+1
=-1/2*(1-cos4x)/2+1
=1/2*cos4x+1/2
-1<=cos4x<=1
0<=1/2cos4x+1/2<=1
所以值域[0,1]