推 CFengY : 有神快拜 吸收知識中 01/22 02:15
推 LiptonTea : 謝謝 01/22 02:34
推 Tiderus : 01/22 14:27
推 Dawsen : 好厲害的恆等式 03/17 22:20
※ 引述《LiptonTea (立頓茶)》之銘言：
: https://i.imgur.com/GkdQ5tR.jpg
: 群組丟出來的題目
: 最多只解到這樣
: https://i.imgur.com/fMsAl4t.jpg
: 麻煩大大幫忙
用三角函數硬爆的話
tan∠BDE = BE/BD = (BE/BC)(BC/BA)(BA/BD)
= tan∠BCE * tan∠BAC * tan∠BDA
= tan 10°* tan 50°* tan 70°
這有一個恆等式可以用:
tan x tan(60°- x) tan(60°+ x) = tan 3x
套用可知 tan∠BDE = tan 30°, 所以 ∠BDE = 30°=> ∠EDA = 40°
--
'Oh, Harry, don't you see?' Hermione breathed. 'If she could have done
one thing to make absolutely sure that every single person in this school
will read your interview, it was banning it!'
---'Harry Potter and the order of the phoenix', P513
--
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