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Thread: Identities help...

  1. November 12th 2008, 04:57 PM #1
    eh501
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    Identities help...

    I'm stuck on this question:

    Prove identity: 1-tanѲsin2Ѳ=cos2Ѳ

    I understand i have to use this formula sin 2A=2sinAcosA but i don't know what to do from there..
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  2. November 13th 2008, 07:30 AM #2
    red_dog
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    We have \cos\theta=\cos^2\theta-\sin^2\theta=2\cos^2\theta-1=1-2\sin^2\theta

    Now, 1-\tan\theta\sin 2\theta=1-\frac{\sin\theta}{\cos\theta}\cdot 2\sin\theta\cos\theta=1-2\sin^2\theta=\cos 2\theta
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