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]
