如图,△ABC中,AD平分∠BAC,CM⊥AD于M,若AB=AD,求证2AM=AB+AC
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如图,△ABC中,AD平分∠BAC,CM⊥AD于M,若AB=AD,求证2AM=AB+AC
证明:
延长AM至E 使得AE=AC,连结EC
∵AD平分∠BAC
∴∠BAD=∠CAE
∵AB=AD ,AE = AC
∴△ABD∽△AEC
∴AB/AC=BD/EC
∵∠BAD = ∠CAD,AB = AD,AC = AE
∴2∠AEC = 2∠B = 2∠ADB = 2∠CDE
∴∠AEC = ∠CDE
∴CD = CE
∵CM⊥DE
∴DM=EM
∴AM
=AD+DM
=(AD+AD+DM+DM)/2
=(AD+AD+MD+ME)/2
=(AD+AE)/2
=(AD+AC)/2
=(AB+AC)/2
延长AM至E 使得AE=AC,连结EC
∵AD平分∠BAC
∴∠BAD=∠CAE
∵AB=AD ,AE = AC
∴△ABD∽△AEC
∴AB/AC=BD/EC
∵∠BAD = ∠CAD,AB = AD,AC = AE
∴2∠AEC = 2∠B = 2∠ADB = 2∠CDE
∴∠AEC = ∠CDE
∴CD = CE
∵CM⊥DE
∴DM=EM
∴AM
=AD+DM
=(AD+AD+DM+DM)/2
=(AD+AD+MD+ME)/2
=(AD+AE)/2
=(AD+AC)/2
=(AB+AC)/2
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