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## #1 2006-03-10 18:45:03

amonjezi
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### solve sin(36)

hi!

sin(36)?
without calculator!

## #2 2006-03-10 19:18:13

krassi_holmz
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### Re: solve sin(36)

IPBLE:  Increasing Performance By Lowering Expectations.

## #3 2006-03-10 22:03:57

MathsIsFun

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### Re: solve sin(36)

Seems to be right!

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

## #4 2006-03-10 22:18:42

krassi_holmz
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### Re: solve sin(36)

Is 36 degrees or 36 rad?
If it's 36 radians, we get:

IPBLE:  Increasing Performance By Lowering Expectations.

amonjezi
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hi krassi
thank u

## #6 2006-03-14 02:49:39

George,Y
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### Re: solve sin(36)

thiat used to be one of my high school excercise.

sin36 = 2 sin18 cos18 = 2 cos72 sin72

2 (sin36)^2=8 (cos72)^2 [1- (cos72)^2]

1 - cos72=8 (cos72)^2  [1- (cos72)^2]

cos72=sin18=x

1-x=8 x^2 (1 - x^2) = 8 x^2 (1-x)(1+x) = (1-x)(8 x^2+8 x^3)

x<1, so 8 x^3+8 x^2-1=0

solve it and get sin18, further get cos18,  sin36 = 2 sin18 cos18

X'(y-Xβ)=0