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Math Help - Help with solving these Trig Identities... Part 3

  1. #1
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    Help with solving these Trig Identities... Part 3

    How do I solve:

    Using Double Angle identities

    1. (2 tan 15) / (1 - tan^2 15)
    2. sin(pi/6)cos(pi/6)
    3. cos^2 (2x) - sin^2 (2x)

    Answers:
    1. [sq.root(3)] / 3
    2. [sq.root(3)] / 4
    3. cos 4x


    Thank you very much for your time! =)
    Last edited by mr fantastic; April 5th 2009 at 04:08 AM. Reason: Questions moved from original thread
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  2. #2
    MHF Contributor red_dog's Avatar
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    Use this:

    1) \frac{2\tan x}{1-\tan^2x}=\tan 2x

    2) \sin\frac{\pi}{6}=\frac{1}{2}, \ \cos\frac{\pi}{6}=\frac{\sqrt{3}}{2}

    3) \cos^2a-\sin^2a=\cos 2a
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