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Thread: Pls help with tough trigo identity!

  1. #1
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    Pls help with tough trigo identity!

    Hi guyz! Can u pls help me with this baffling trigo identity?
    If tan^2 A -2 tan^2 B = 1,
    show that cos^2 B = 2 cos^2 A.
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  2. #2
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    Re: Pls help with tough trigo identity!

    $\sec^2{a}-1-2(\sec^2{b}-1)=1$

    $\sec^2{a}-2\sec^2{b}=0$

    $\dfrac{1}{\cos^2{a}}=\dfrac{2}{\cos^2{b}}$

    finish by cross-multiplying ...
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  3. #3
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    Re: Pls help with tough trigo identity!

    Thanx aplenty! Why didn't I think of that?
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