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Math Help - Proving a Trig Identity

  1. #1
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    Proving a Trig Identity

    Hi there, I need help proving a trig ident. steps plz,

    cos2x= (1-tan^2(x))
    ..........(1+tan^2(x))

    tan^2(x) is tan(x)^2....if u don't understand my writing

    Please, any help is appreciated, thanks
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  2. #2
    Eater of Worlds
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    You can use the identity:

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

    It'll fall into place.
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  3. #3
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    thanks so much
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  4. #4
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    Quote Originally Posted by compuwiz View Post
    cos2x= (1-tan^2(x))
    ..........(1+tan^2(x))
    1+\tan^2x=\sec^2x. (Basic identity.)

    Now split the original fraction into two sums:

    \frac{{1 - \tan ^2 x}}<br />
{{\sec ^2 x}} = \cos ^2 x - \sin ^2 x.

    And this is exactly equal to double-angle cosine formula.
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