f(x)=（cosx）^4-(sinx)^4-2sinxcosx
={(cosx)^2+(sinx)^2}*{(cosx)^2-(sinx)^2}-2sinxcosx
=1*{(cosx)^2-(sinx)^2}-2sinxcosx
=cos2x-sin2x
=√2cos(2x+45°)
最小正周期 T=2π/2=π
x属于[0,90度],
当x=0时最大,最大值为√2*√2/2=1
当x=67.5°时最小 ,最小值为 √2*（-1）=-√2