※ 引述《math365 (math365)》之銘言：
: https://ibb.co/1YRBZmRJ
: 國中相似形的單元，麻煩大家幫忙一下，謝謝
過D分別做BA和BE的平行線交AC於J、H => EH = 5
J、H對BC的垂足分別為D'、F'
令AG = GD = a，JH = 5r，HC = t => r = t/(5 + t)
△HF'C ~ △ADC => 6r/t = 2a/(10 + t) => a = 3(10 + t)/(t + 5)
DA^2 = AJ * AC => (2a)^2 = (10 - 5r)(10 + t) => t = 11/5
=> EC = 5 + t = 36/5
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