设 sinx = a cosx = b
ab = 1/2sin2x
y=sinx的四次方+cosx的四次方
= a^4 + b^2
= a^4 + b^2 + 2a^2b^2 - 2a^2b^2
= (a^2 + b^2)^2 - 2a^2b^2
= 1 - 2（1/2sin2x）^2
= 1- 1/2*(1-cos4x)/2
= 3/4 + cos4x/4
所以周期 = 2π/4 = π/2