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People's education edition, the first volume of the second day, page 83 10 answer.
Prove that ∫ob and OC are divided by ∠ ABC, ∠ ACB, ∴∠ MBO = ∠ OBC, ∠ NCO = ∠ OCB respectively.

∫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.