Math Is Fun Forum
You are not logged in.
- Topics: Active | Unanswered
#1 2006-01-12 07:13:17
- RauLiTo
- Member
- From: Bahrain
- Registered: 2006-01-11
- Posts: 142
help help
how you doing ?
can you prove that :
cos 144 + cos 72 = - 1 / 2 ?
waiting for the answer like hell !
ImPo$$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...
Offline
#2 2006-01-12 20:17:58
- RauLiTo
- Member
- From: Bahrain
- Registered: 2006-01-11
- Posts: 142
Re: help help
no one can help me guys ?
ImPo$$!BLe = NoTH!nG
Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...
Offline
#3 2006-01-12 22:18:08
- RickyOswaldIOW
- Member
- Registered: 2005-11-18
- Posts: 212
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.
Offline
#4 2006-01-12 22:33:31
- krisper
- Member
- Registered: 2005-12-20
- Posts: 19
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 (2006-01-14 22:56:41)
Humankind's inherent sense of right and wrong cannot be biologically explained.
Offline
#5 2006-01-12 23:24:46
- RauLiTo
- Member
- From: Bahrain
- Registered: 2006-01-11
- Posts: 142
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 ...
Offline
#6 2006-01-12 23:29:54
- MathsIsFun
- Administrator
- Registered: 2005-01-21
- Posts: 7,711
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
Offline
#7 2006-01-13 02:01:55
- kempos
- Member
- Registered: 2006-01-07
- Posts: 77
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 (2006-01-13 02:02:52)
Offline
#8 2006-01-13 02:03:26
- RickyOswaldIOW
- Member
- Registered: 2005-11-18
- Posts: 212
Re: help help
y? cos
TEHE
Last edited by rickyoswaldiow (2006-01-13 02:03:37)
Aloha Nui means Goodbye.
Offline
#9 2006-01-13 23:21:22
- krisper
- Member
- Registered: 2005-12-20
- Posts: 19
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 (2006-01-13 23:21:58)
Humankind's inherent sense of right and wrong cannot be biologically explained.
Offline