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證明:在△ABC中,∵AB=AC,∠BAC=120°,∴∠B=∠C=30°,又∵AD⊥AC,∴∠DAC=90°,∵∠C=30°∴CD=2AD,∠BAD=∠B=30°,∴AD=DB,∴BC=CD+BD=AD+DC=AD+2AD=3AD.
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