答案为√3
∵三个内角A,B,C成等差数列
∴∠B=180°/3 = 60°
∵D是BC中点
∴BD = BC/2 = 4/2 = 2
用余弦定理可求AD
AD^2 = AB^2 + BD^2 - 2*AB*BD*cos∠B
= 1+ 4 - 2*1*2*1/2
= 3
∴AD = √3