Re: [中學]
※ 引述《gunhello (資深動感超人)》之銘言：
: https://iiil.io/P8Yw
: 拜託各位高手，可以幫我解決這個幾何問題嗎？謝謝。
純幾何解法：
過E做AB的平行線交BC於C'
過D做BC的平行線交AC於D'，過C做BA的平行線，此兩線相交於H
BH交AC於I
CD垂直BH於O
D對AD'做垂足O'
∠CIB = 60 = ∠DIE
△IDO' = △ICO => OC = O'D
=> △OCB = △O'DE
=> ∠DEO' = ∠CBO = 40
令∠ADE = a => ∠DEB = ∠BEC' = a
=> ∠DEO' = 2a + 20 = 40
=> a = 10
=> ∠DEF = 2a + 10 = 30度
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