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Abstract Algebra Question dealing with Constructible Numbers

  1. Nov 6, 2011 #1
    1. The problem statement, all variables and given/known data

    Let c=cos(2pi/5). It can be shown that (4c^2)+(2c)-1=0. Use this fact to prove that a 72degree angle is constructible.

    2. Relevant equations

    3. The attempt at a solution

    I can see that using the equation and what c equals that you get the statement 0=0 and I know that 2pi/5 relates to 72degree angle on the unit circle but I have no idea how to formally prove it. Also can equate cos(2pi/5) but don't see how that would help either.
  2. jcsd
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