假设A(x1,y1),B(x2,y2),C(x3,y3)(x1+x2)/2=3 (y1+y2)/2=-2(x2+x3)/2=5 (y2+y3)/2=-4(x1+x3)/2=-1 (y1+y3)/2=-8=>x1=-3 y1=-6=>AD=根号[(-3-5)^2+(-8+4)^2]=4根号5
