因为y=3/2(cosx/2+根号3sinx/2)
=3（1/2cosx/2+根号3/2sinx/2)
=3(cosx/2cosπ/3+sinx/2sinπ/3)
=3cos(x/2-π/3)
所以周期是T=2π/（1/2）=4π,振幅A=3,初相是-π/3,
由2kπ-π=