直线ab的斜率=(2+1)/(2-1)=3
cd⊥ab;则cd的斜率=-1/3
cb的斜率=(2-0)/(2-3)=-2=ad的斜率
设d点坐标为(x,y)
则:(y-0)/(x-3)=-1/3;
(y+1)/(x-1)=-2
x=0,y=1
即d点坐标为(0,1)