y=cos2（平方）x+sin2x
=(cos2x+1)/2+sin2x
=√5/2sin(2x+θ)+1/2
最小正周期为π
单调增区间
[kπ/2-π/4-θ,kπ/2+π/4-θ]
其中θ=arccos(2/√5)