y=(sinxcospi/6+cosxsinpi/6)(sinxcospi/6-cosxsinpi/6)
=(sinxcospi/6)^2-(cosxsinpi/6)^2
=3/4*(sinx)^2-1/4*(cosx)^2
=3/4*(1-cos2x)/2-1/4*(1+cos2x)/2
=3/4-3/8cos2x-1/4-1/8*cos2x
=-1/2*cos2x+1/2
所以T=2pi/2=pi