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Math Help - Double angle identities

  1. #1
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    Double angle identities

    Use the identity cos(2x)=2cos^2(x)-1 to form a quadratic equation in terms of cos(pi/5), hence find the exact value of cos(pi/5).

    Any help please? I tried sub (pi/5) in but doesn't seemed to work.... Thanks.....
    Last edited by mr fantastic; October 3rd 2009 at 11:41 PM. Reason: Fixed an ambiguity
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  2. #2
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    Quote Originally Posted by anonymous_maths View Post
    Use the identity cos(2x)=2cos^2(x)-1 to form a quadratic equation in terms of cos(pi/5), hence find the exact value of cos(pi/5).

    Any help please? I tried sub (pi/5) in but doesn't seemed to work.... Thanks.....
    It can't be done using that identity. Either the wrong identity has been given or the wrong angle to found has been given.
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  3. #3
    Senior Member pacman's Avatar
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    your supposed identity is unknown to me . . . .

    this may help, Surds-prove a formula for sin36? - Yahoo! Answers

    steiner1745 says something about that identity but is not. .

    cos 36 = (1 + √5)/4
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