Discussion about math, puzzles, games and fun. Useful symbols: ÷ × ½ √ ∞ ≠ ≤ ≥ ≈ ⇒ ± ∈ Δ θ ∴ ∑ ∫ π -¹ ² ³ °

You are not logged in.

- Topics: Active | Unanswered

**RauLiTo****Member**- From: Bahrain
- Registered: 2006-01-11
- Posts: 142

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

**RauLiTo****Member**- From: Bahrain
- Registered: 2006-01-11
- Posts: 142

no one can help me guys ?

ImPo$$!BLe = NoTH!nG

Go DowN DeeP iNTo aNyTHinG U WiLL FinD MaTHeMaTiCs ...

Offline

**RickyOswaldIOW****Member**- Registered: 2005-11-18
- Posts: 212

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

**krisper****Member**- Registered: 2005-12-20
- Posts: 19

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

**RauLiTo****Member**- From: Bahrain
- Registered: 2006-01-11
- Posts: 142

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

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,626

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

**kempos****Member**- Registered: 2006-01-07
- Posts: 77

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

**RickyOswaldIOW****Member**- Registered: 2005-11-18
- Posts: 212

y? cos

TEHE

*Last edited by rickyoswaldiow (2006-01-13 02:03:37)*

Aloha Nui means Goodbye.

Offline

**krisper****Member**- Registered: 2005-12-20
- Posts: 19

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