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