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February 1, 2015

February 1, 2015

Posted by **ody** on Wednesday, May 28, 2008 at 9:38pm.

- trig -
**drwls**, Wednesday, May 28, 2008 at 11:10pmSince that is a very small angle, the radian approximation

sin theta = theta (in radians)

can be used.

18' = 18/60 degree = 0.3000 degrees

= 0/3*(pi/180)= pi/600 radians

= 0.00523599..

The true sine of the angle is very nearly the same. The exact value is 0.00523596..

- trig -
**Reiny**, Thursday, May 29, 2008 at 6:16amI have seen this question as :

find the exact value of sin 18º

18 = 1/2 of 36

Start with a regular pentagon and draw several of the diagonals.

You will see an abundance of angles of 36º,72º and 108º in similar triangles,

and the diagonals intersect each other in the "golden ratio" which is (1+√5)/2

using the cosine law we can show that

cos 36º = (1+√5)/4 which is 1/2(golden ratio)

cos 36º = 1 - 2sin^2 18º

substituting and solving for sin 18º

I got sin 18º = √[(3-√5)/8]

- trig -
**drwls**, Thursday, May 29, 2008 at 2:49pmThe question still shows up as "18 minutes" on my (Mac) computer but the symbol for degrees appears in Reiny's answer. C'est la vie

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