已知2sinθ= -3 cotθ 求cosθ?
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數學~三角單元~已知2sinθ=-3cotθ 求cosθ?
2010-12-01 10:07 pm
回答 (3)
2010-12-01 10:24 pm
✔ 最佳答案
2sinθ=-3cotθ2sinθ=-3(cosθ/sinθ)
2(sinθ)^2=-3cosθ
2*[1-(cosθ)^2]+3cosθ=0
-2(cosθ)^2+3cosθ+2=0
2(cosθ)^2-3cosθ-2=0
(cosθ-2)(2cosθ+1)=0
cosθ=2>1(不合)
cosθ= -1/2
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2010-12-01 10:25 pm
2sinθ= -3cotθ 2 (sinθ)^2= -3cosθ 2(1-(cosθ) ^2)=-3 cosθ令cosθ=c2c^2 -3c -2=0c= -1/2 , 2 (不合)信欣茗
http://tw.myblog.yahoo.com/wang620628/article?mid=447&next=446&l=f&fid=5
好笑的喔
http://tw.myblog.yahoo.com/wang620628/article?mid=447&next=446&l=f&fid=5
好笑的喔
2010-12-01 10:20 pm
2sinθ = -3cotθ
2sinθ = -3(cosθ/sinθ)
2sin²θ = -3cosθ
2(1 - cos²θ) = -3cosθ
2 - 2cos²θ = -3cosθ
2cos²θ - 3cosθ - 2 = 0
(2cosθ + 1)(cosθ - 2)= 0
cosθ = -1/2 或 cosθ = 2 (捨棄)
所以 cosθ = -1/2
2sinθ = -3(cosθ/sinθ)
2sin²θ = -3cosθ
2(1 - cos²θ) = -3cosθ
2 - 2cos²θ = -3cosθ
2cos²θ - 3cosθ - 2 = 0
(2cosθ + 1)(cosθ - 2)= 0
cosθ = -1/2 或 cosθ = 2 (捨棄)
所以 cosθ = -1/2
參考: 土扁
收錄日期: 2021-04-13 17:40:24
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20101201000015KK03241