Take AB side as X axis, establish rectangular coordinate system, point A is coordinate 0, point B is coordinate (4,0), and point C is coordinate (x, y), which is not on AB straight line. D is the midpoint of the BC side,

Then that coordinate of point d is (x+4/2, y/2),

Then AD=√[((x+4)/2)? +(y/2)？ ]=3

Simplify,

(x+4)？ +y？ =36 is a circle with (-4,0) as the center and 6 as the radius.

And，(x，y)≦(2，0) and (- 10，0)。