证明：延长AM交CB延长线于E,延长AN交BC延长线于F∵BM平分∠ABE,BM⊥AM∴AM＝EM,AB＝BE∴AM＝AE/2∵CN平分∠ACF,CN⊥AN∴AN＝FN,AC＝CF∴AN＝AF/2∴MN∥EF∴MN/EF＝AM/AE∴MN＝EF/2∵EF＝BE+BC+CF∴EF＝AB+BC+AC∴MN...