y=Sin3x-Sin(π/2-3x)
=2Cos[(3x+π/2-3x)/2]Sin[(3x+π/2+3x)/2]
=2Cos(π/4)Sin(3x+π/4)
=√2Sin(3x+π/4)
所以3T=2π,
T=2π/3
最小正周期2π/3