(setq A (point 3 3))
(setq B (point 5 8))
(setq C (point 7 3))
(setq triangle_a (triangle A B C))
(setq O (circumcenter triangle_a))

triangle_a
O
(lineseg (midpoint A B) O)
(lineseg (midpoint B C) O)
(lineseg (midpoint A C) O)