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Math Help - trig - prove identities

  1. #1
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    trig - prove identities

    prove so L.S. = R.S.
    sin8A = 8sinAcosAcos2Acos4A


    please help have test tommorow

    thanks
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  2. #2
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    Hello, getem j!

    We're expected to know this identity: . \sin2\theta \:=\:2\cdot\sin\theta\cdot\cos\theta


    Prove: . \sin8A \:= \:8\cdot\sin A\cdot\cos A\cdot\cos2A\cdot\cos4A

    We have: . \sin8A \;=\;2\cdot\underbrace{\sin4A}\cdot\cos4A

    . . . . . . . . . =\; 2\overbrace{(2\cdot\sin2A\cdot\cos2A)}\cdot\cos4A

    . . . . . . . . . =\;4\cdot\underbrace{\sin2A}\cdot\cos2A\cdot\cos4A

    . . . . . . = \;4\cdot\overbrace{(2\cdot\sin A\cdot\cos A)}\cdot\cos2A\cdot\cos4A

    . . . . . . = \;8\cdot\sin A\cdot\cos A\cdot\cos2A\cdot\cos4A

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  3. #3
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    wow thank you so much , your a life saver
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