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

You are not logged in.

## #1 2006-03-09 19:45:03

amonjezi
Member
Registered: 2006-03-09
Posts: 9

### solve sin(36)

hi!

sin(36)?
without calculator!

Offline

## #2 2006-03-09 20:18:13

krassi_holmz
Real Member
Registered: 2005-12-02
Posts: 1,905

### Re: solve sin(36)

IPBLE:  Increasing Performance By Lowering Expectations.

Offline

## #3 2006-03-09 23:03:57

MathsIsFun
Registered: 2005-01-21
Posts: 7,658

### Re: solve sin(36)

Seems to be right!

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

Offline

## #4 2006-03-09 23:18:42

krassi_holmz
Real Member
Registered: 2005-12-02
Posts: 1,905

### Re: solve sin(36)

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

IPBLE:  Increasing Performance By Lowering Expectations.

Offline

## #5 2006-03-11 01:44:13

amonjezi
Member
Registered: 2006-03-09
Posts: 9

hi krassi
thank u

Offline

## #6 2006-03-13 03:49:39

George,Y
Member
Registered: 2006-03-12
Posts: 1,306

### 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

Offline