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 A019827 Decimal expansion of sin(Pi/10) (angle of 18 degrees). 16

%I

%S 3,0,9,0,1,6,9,9,4,3,7,4,9,4,7,4,2,4,1,0,2,2,9,3,4,1,7,1,8,2,8,1,9,0,

%T 5,8,8,6,0,1,5,4,5,8,9,9,0,2,8,8,1,4,3,1,0,6,7,7,2,4,3,1,1,3,5,2,6,3,

%U 0,2,3,1,4,0,9,4,5,1,2,2,4,8,5,3,6,0,3,6,0,2,0,9,4,6,9,5,5,6,8

%N Decimal expansion of sin(Pi/10) (angle of 18 degrees).

%C Decimal expansion of cos(2*Pi/2) (angle of 72 degrees).

%C Also the imaginary part of i^(1/5). - _Stanislav Sykora_, Apr 25 2012

%C One of the two roots of 4x^2 + 2x - 1 (the other is the sine of 54 degrees times -1). - _Alonso del Arte_, Apr 25 2015

%C This is the height h of the isosceles triangle in a regular pentagon inscribed in a unit circle, formed by a diagonal as base and two adjacent radii. h = cos(2*Pi/5) = sin(Pi/10). - _Wolfdieter Lang_, Jan 08 2018

%C Quadratic number of denominator 2 and minimal polynomial 4x^2 + 2x - 1. - _Charles R Greathouse IV_, May 13 2019

%H Zak Seidov, <a href="/A019827/b019827.txt">Table of n, a(n) for n = 0..999</a>

%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Exact_trigonometric_constants">Exact trigonometric constants</a>

%F Equals (sqrt(5) - 1)/4 = (phi - 1)/2 = 1/(2*phi), with phi from A001622.

%F Equals 1/(1 + sqrt(5)). - _Omar E. Pol_, Nov 15 2007

%e 0.30901699437494742410229341718281905886015458990288143106772431135263...

%t RealDigits[Sin[18 Degree], 10, 108][[1]] (* _Alonso del Arte_, Apr 20 2015 *)

%o (PARI) sin(Pi/10) \\ _Charles R Greathouse IV_, Feb 03 2015

%o (PARI) polrootsreal(4*x^2 + 2*x - 1)[2] \\ _Charles R Greathouse IV_, Feb 03 2015

%Y Cf. A001622, A019845, A019863.

%K nonn,cons,easy

%O 0,1

%A _N. J. A. Sloane_

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Last modified February 27 15:16 EST 2020. Contains 332307 sequences. (Running on oeis4.)