答：
0<=x<=2π
0<=x/2<=π
-π/3<=x/2-π/3<=2π/3
所以：cos(2π/3)<=cos(x/2-π/3)<=cos0
所以：-1/2<=y<=1
所以：y=cos(x/2-π/3)的值域为[-1/2,1]