∫MN∨BC, Mn passes through point O, ∴∠ Mob = ∠ OBC, ∠ NOC = ∠ OCB, ∴ Mo = MB, No = NC.
∴△ circumference of ∴△AMN = AM+Mn+An = AM+Mo+NO+An = AM+MB+NC+An = AB+AC.