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Proving this trigonometric identity

  1. May 26, 2012 #1
    1. The problem statement, all variables and given/known data
    Show that :

    1 + cos(2∏/5)= 2 cos(∏/5)

    2. Relevant equations

    cos(2x) = cos^2(x)-sin^2(x)
    cos^2(x)+sin^2(x) = 1

    3. The attempt at a solution

    I have tried using the two formulas above but i couldn't show the required result.
     
  2. jcsd
  3. May 26, 2012 #2

    ehild

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    It is not true. ∏/5=36° , 1+cos(72°)≠2cos(36°)

    ehild
     
  4. May 26, 2012 #3
    Looks like the identity should have been:

    1 + cos(2∏/5) = 2 cos^2(∏/5)
     
  5. May 26, 2012 #4

    SammyS

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    Hello sacscale. Welcome to PF !


    I believe that you are correct !
     
  6. Jun 16, 2012 #5
    Yes but the issue remains, how do we prove it ?
     
  7. Jun 16, 2012 #6

    Mark44

    Staff: Mentor

    Assuming you are trying to prove 1 + cos(2##\pi##/5) = cos2(##\pi##/5) rather than what you originally posted, rewrite the left side using the double angle identity.
     
  8. Jun 25, 2012 #7
    alright, got it
    thnx
     
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