令a=sinx,b=cosx,则a^2+b^2=1
分子=a^4+b^4+a^2b^2=a^4+2a^2b^2+b^4-a^2b^2
=(a^2+b^2)^2-a^2b^2
=1-a^2b^2
=1-(sinxcosx)^2
=1-(1/4)(sin2x)^2
=[1+(1/2)sin2x][1-(1/2)sin2x]
分母=2[[1-(1/2)sin2x]
所以f(x)=(1/2)+(1/4)sin2x
所以T=2π/2=π
-1<=sin2x<=1
-1/4<=(1/4)sin2x<=1/4
1/2-1/4<=1/2+(1/4)sin2x<=1/2+1/4
所以最大值=3/4,最小值=1/4