The circumcenter has equal distance to each of the three vertices. Suppose the coordinates of the circumcenter O are (a,b). Then OD=\sqrt((a-1)^2+(b-3)^2), OE=\sqrt(a-8)^2+(b-3)^2), OF=\sqrt((a-1)^2+(b+5)^2). Let OD=OE=OF, solve the equation. x=9/2, y=-1. O(9/2, -1).
