π/12<=x<=π/2
π/6<=2x<=π
π/3<=2x+π/6<=7π/6
sin在(π/3,π/2)是增函数
(π/2,7π/6)是减函数
所以π/2时最大,7π/6时最小
sinπ/6=1,sin7π/6=-1/2
所以值域[-1,2]