如图：分别过点A、D作CD和BC的垂线交各自直线或延长线于点E、F

          很容易得：AC=CD=√3km,  AE=3/2,

           ∵ 在Rt△AED中,角ADE=30°

       ∴AD=3  

      同理可得：BD=√2

          ∴在△ABD中,由余弦定理：得：AB²=AD²+BD²-2*AD*BD*cos45°

                                       =3²+(√2)²-2*3*(√2)*cos45°=5

                 所以,目标a,b的距离=根号5