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Prove that cos2x=(1-tan^2x)/(1+tan^2x)

I can't find any identities that can help me with this. I'm really confused

  • Math -

    1 + tan^2 x= (cos^2 x+ sin^2 x)/cos^2 x = 1/cos^2 x

    Therefore
    =(1-tan^2x)/(1+tan^2x)= cos^2x - sin^2x
    = cos 2x

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