∵-π/6≤x≤13π/12
∴0≤2x+π/3≤5π/2
2sin(2x+π/3)-1=a
sin(2x+π/3)=(a+1)/2
令t=2x+π/3∈[0,5π/2]
sint=(a+1)/2
（画出sint在[0,5π/2]的函数图像）
(a+1)/2=1或-1≤(a+1)/2