本區搜索:
Yahoo!字典

投票項目 ( 單選 ) 參與人數 4

1. Let x=cos(pi/5)
 

2 (50.00%)

2. Let x=cos(3pi/5)
 

2 (50.00%)

打印

[M2] How would you find cos(3pi/5)?

[隱藏]

How would you find cos(3pi/5)?

This is referring to the 2016 DSE question.As the question is asking for cos(3pi/5), naturally the substitution x=cos(3pi/5) should be attempted.(It is not difficult to show that 4x^3+2x^2-3x-1=(4x^3-3x)+(2x^2-1)=cos(9pi/5)+cos(6pi/5)=cos(pi/5)-cos(pi/5)=0.)

It seems that some candidates prefer to let x=cos(pi/5) and then use double/triple angle formula to get back x=cos(3pi/5).
I am a bit curious what is the motivation behind your choice. Feel free to share your opinion.

[ 本帖最後由 桃子 於 2016-5-2 07:21 PM 編輯 ]
   

TOP

重要聲明:小卒資訊論壇 是一個公開的學術交流及分享平台。 論壇內所有檔案及內容 都只可作學術交流之用,絕不能用商業用途。 所有會員均須對自己所發表的言論而引起的法律責任負責(包括上傳檔案或連結), 本壇並不擔保該等資料之準確性及可靠性,且概不會就因有關資料之任何不確或遺漏而引致之任何損失或 損害承擔任何責任(不論是否與侵權行為、訂立契約或其他方面有關 ) 。