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過B作BE垂直於AD,過C作CF垂直於AD. 因為∠BAD=∠CAD,∠BEA=∠CFA=90度 所以三角形BAE相似於三角形CAF→AB/AC=BE/CF① 因為∠BDE=∠CDF,∠BDE=∠CDF=90度 所以三角形BDE相似於三角形CDF→BE/CF=BD/CD② 由①②得,AB/AC=BD/CD
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