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February 29th, 2012, 12:36 AM  #1 
Newbie Joined: Feb 2012 Posts: 17 Thanks: 0  De Moivre's Theorem
how to find the exact value sin??2?/5? using Demoivers Theorem algebraically ?

February 29th, 2012, 01:31 AM  #2 
Senior Member Joined: Jul 2010 From: St. Augustine, FL., U.S.A.'s oldest city Posts: 12,211 Thanks: 520 Math Focus: Calculus/ODEs  Re: De Moivre's Theorem We may then consider the roots of the equation: Now simply choose the complex root a + bi where a and b are positive, i.e., in the first quadrant of the complex plane. This will come from the quadratic factor whose coefficient of the linear term is negative: and choosing the root we find: Thus: 

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