θ=w(因为很难打)
sinw-cosw=m
(sinw-cosw)^2=m^2
1-2sinwcosw=m^2
sinwcosw=(1-m^2)/2
sin^4w+cos^4w
=(sin^2w+cos^2w)^2-2(sinwcosw)^2
=1-2(sinwcosw)^2
=1-2(1-m^2)^2/4
=(2m^2-m^4+1)/2