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## #1 2006-01-13 06:13:17

RauLiTo
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### help help

how you doing ?

can you prove that :

cos 144 + cos 72 = - 1 / 2 ?

waiting for the answer like hell !

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

## #2 2006-01-13 19:17:58

RauLiTo
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### Re: help help

no one can help me guys ?

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

## #3 2006-01-13 21:18:08

RickyOswaldIOW
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### Re: help help

There are a good few people on this board who will help you, but you must have a little patience.  They are probably in bed

Aloha Nui means Goodbye.

## #4 2006-01-13 21:33:31

krisper
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### Re: help help

cos144 + cos72 = 2cos(144+72)/2*cos(144-72)/2=2cos108*cos36=
2cos(180-72)*cos36=-2cos72*cos36=(-cos72*2cos36*sin36)/sin36=
(-cos72*sin72)/sin36=(-1/2)*(sin144)/sin36=(-1/2)*(sin144)/(sin(180-36))=
(-1/2)(sin144)/(sin144)=-1/2

Last edited by krisper (2006-01-15 21:56:41)

Humankind's inherent sense of right and wrong cannot be biologically explained.

## #5 2006-01-13 22:24:46

RauLiTo
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### Re: help help

sirry rickyoswaldiow i didn't know that it was night
krisper thank you alot alot man

ImPo\$\$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

## #6 2006-01-13 22:29:54

MathsIsFun
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### Re: help help

Well done krisper, maybe you could tell us about yourself in Introductions

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman

## #7 2006-01-14 01:01:55

kempos
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### Re: help help

cos144 + cos72 = 2cos(144+72)/2*cos(144-72)/2=2cos108*cos36=
!!!!!!!!!!2cos(180-108)!!!!!!!!!!!*cos36=-2cos72*cos36=(-cos72*2cos36*sin36)/sin36=
(-cos72*sin72)/sin36=(-1/2)*(sin144)/sin36=(-1/2)*(sin144)/(sin(180-36))=
(-1/2)(sin144)/(sin144)=-1/2

it should be 2cos(180-72)!!!

Last edited by kempos (2006-01-14 01:02:52)

## #8 2006-01-14 01:03:26

RickyOswaldIOW
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### Re: help help

y? cos
TEHE

Last edited by rickyoswaldiow (2006-01-14 01:03:37)

Aloha Nui means Goodbye.

## #9 2006-01-14 22:21:22

krisper
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### Re: help help

kempos you are correct
It should be:

cos144 + cos72 = 2cos(144+72)/2*cos(144-72)/2=2cos108*cos36=
2cos(180-72)*cos36=-2cos72*cos36=(-cos72*2cos36*sin36)/sin36=
(-cos72*sin72)/sin36=(-1/2)*(sin144)/sin36=(-1/2)*(sin144)/(sin(180-36))=
(-1/2)(sin144)/(sin144)=-1/2

Last edited by krisper (2006-01-14 22:21:58)

Humankind's inherent sense of right and wrong cannot be biologically explained.

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