y=2cos(2x+π/3)
-π/6≤x≤π/6
-π/3≤2x≤π/3
0≤2x+π/3≤2π/3
-1/2≤cos(2x+π/3)≤1
-1≤2cos(2x+π/3)≤2
所以值域是[-1,2]