y=(sinx)^4+(cosx)^4
=[(sinx)^2+(cosx)^2]^2-2(sinx)^2(cosx)^2
=1-2(sinx)^2(cosx)^2
=1-4(sinx)^2(cosx)^2/2
=1-(sin2x)^2/2
=(cos4x+3)/4
所以周期是π/2
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