香港寵物小精靈村落 論壇

標題: 又有關trigo [打印本頁]

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作者: ting    時間: 9/4/2007 03:08 PM
標題: 又有關trigo
請問cot90是0還是undefined呢?


3^0.5/tanA-2cosA=0

(3^0.5-2sinA)/tanA=0

3^0.5-2sinA=0

sinA=3^0.5/2

A=60

但答案是90和60，其實以上ge步驟邊度出錯?

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作者: sapphire    時間: 9/4/2007 04:27 PM
tan90∘→∞
cot90∘=1/tan90∘→0

3^0.5/tanA-2cosA=0
(3^0.5-2sinA)/tanA=0
(3^0.5-2sinA)cotA=0
3^0.5-2sinA=0 or cotA=0
A=60∘ or 90∘

u should not multiply both sides by tanA as u will miss the root 90.
note that if the question has not state A lies in the 1st quad., there are 2 more roots.
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however, i think it is not good to do in this way as it contains limit.

[3^(1/2)]/tanA-2cosA=0
cosA[3^(1/2)/sinA-2]=0
cosA=0 or 2sinA=3^(1/2)
A=60∘ or 90∘

[ 本帖最後由 sapphire 於 9/4/2007 16:45 編輯 ]

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