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ÊÁѤÃÊÁÒªÔ¡ ¤ÙèÁ×Í¡ÒÃãªé ÃÒª×èÍÊÁÒªÔ¡ »¯Ô·Ô¹ ¤é¹ËÒ ¢éͤÇÒÁÇѹ¹Õé ·Óà¤Ã×èͧËÁÒÂÍèÒ¹·Ø¡ËéͧáÅéÇ

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  #1  
Old 22 ÊÔ§ËÒ¤Á 2016, 11:07
i^i i^i äÁèÍÂÙèã¹Ãкº
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ÍÂÒ¡·ÃÒºÇèÒàÃÒ¨ÐÊÃéÒ§ÊÁ¡Ò÷ÕèÁÕ cos(pi/9) , cos(2pi/9) ,cos(4pi/9)
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  #2  
Old 24 ÊÔ§ËÒ¤Á 2016, 23:32
gon's Avatar
gon gon äÁèÍÂÙèã¹Ãкº
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ÍéÒ§ÍÔ§:
¢éͤÇÒÁà´ÔÁà¢Õ¹â´Â¤Ø³ i^i View Post
ÍÂÒ¡·ÃÒºÇèÒàÃÒ¨ÐÊÃéÒ§ÊÁ¡Ò÷ÕèÁÕ cos(pi/9) , cos(2pi/9) ,cos(4pi/9)
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⨷Âìáµè§àͧËÃ×Íà»ÅèÒ¤ÃѺ à¾ÃÒзÑé§ÊÒÁ¤èÒ¹Ñé¹äÁèà¢éҾǡ¡Ñ¹àÅÂ

µÑÇááÊÍ´¤Åéͧ cos 3A = 1/2, ÊͧµÑÇËÅѧÊÍ´¤Åéͧ cos 3A = -1/2

¶éҵͺẺ·×èÍ æ ¡ç (x - cos(pi/9))(cos(2pi/9))(x-cos(4pi/9)) = 0 «Ö觶éÒ¡ÃШÒÂÍÍ¡ÁÒ¨ÐäÁèÊÇÂ

¶éÒ¨ÐãËéÊÇ µéͧàÅè¹ÊÁ¡ÒáÓÅѧˡàÅ cos 3A = 1/2 ¡Ñº cos 3A = -1/2 ÁÒÂÓÃÇÁ¡Ñ¹ ä´éÊÁ¡Òà $cos ^2 3A = 1/4$

«Ö觡ÃШÒÂáÅéǨÐä´éà»ç¹ $64x^6-96x^4+36x^2-1=0 , x = \cos A$
µÍº¾ÃéÍÁÍéÒ§ÍÔ§¢éͤÇÒÁ¹Õé
  #3  
Old 25 ÊÔ§ËÒ¤Á 2016, 20:44
i^i i^i äÁèÍÂÙèã¹Ãкº
¡ÃкÕèäÇ
 
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¢Íº¤Ø³ÁÒ¡¤ÃѺ
»Å. ¤×ͨÃÔ§æ¼Áµéͧ¡Ò÷ÃÒºÇèÒ¨ÐÊÃéÒ§ÊÁ¡Ò÷ÕèÁÕ cos(20),cos(40),cos(80) à»ç¹ÃÒ¡¢Í§ÊÁ¡Òà ä´éÍÂèÒ§ääÃѺ
µéͧ¢Íâ·É¤Ø³ gon ´éǹФÃѺ ·Õè¼ÁÊ×èͤÇÒÁËÁÒ¼Դæä»ÍФÃѺ
µÍº¾ÃéÍÁÍéÒ§ÍÔ§¢éͤÇÒÁ¹Õé
  #4  
Old 20 ¸Ñ¹ÇÒ¤Á 2016, 14:31
tngngoapm's Avatar
tngngoapm tngngoapm äÁèÍÂÙèã¹Ãкº
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¤ÇÒÁ¨Ãԧ⨷Âì¢é͹ÕéÊǹФÃѺ ¤×Í ÊÁ¡ÒáÓÅѧÊÒÁ·ÕèÁÕÃÒ¡à»ç¹ cos20,cos40,cos80¹èÐÁÕ¤ÃѺáµèÍÂèÒ§·ÕèÇèÒÊÑÁ»ÃÐÊÔ·¸Ôì¨ÐäÁèŧµÑÇ
áµèÅͧÊѧࡵ´Õ´Õ¶éÒà»ÅÕ蹤èÒÃÒ¡à»ç¹
(-cos20),cos40,cos80¨Ð·ÓãËéä´éÊÁ¡ÒáÓÅѧÊÒÁ·ÕèÁÕÊÑÁ»ÃÐÊÔ·¸Ôìà»ç¹¨Ó¹Ç¹àµçÁ¾Í´ÕàŤ×Í $8x^{3}-6x+1=0$
µÍº¾ÃéÍÁÍéÒ§ÍÔ§¢éͤÇÒÁ¹Õé
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