f(0)=0+0-a=-√3
a=√3
f(x)=2√3(1-cos2x)/2+sin2x-√3
=sin2x-√3cos2x
=2sin(2x-π/3)
-π/3<=2x-π/3<=2π/3
所以最大是2sinπ/2=2
最小是2sin(-π/3)=-√3
所以值域是[-√3,2]