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Math Help - Finding functions of g(x) and f(x)

  1. #1
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    Finding functions of g(x) and f(x)

    I'm really stuck on this question, any help would be appreciated.

    If f(3)=-2 and f'(3)=5, find g'(3) if: g(x)=(3x^2)-5f(x)

    the answer is: -7

    thanks in advance
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  2. #2
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    Re: Finding functions of g(x) and f(x)

    Hey JellyOnion.

    Hint: Try differentiating g(x) by noting that d/dx(h(x) + j(x)) = h'(x) + j'(x) and substitute in f'(3) where appropriate.
    Thanks from JellyOnion
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  3. #3
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    Re: Finding functions of g(x) and f(x)

    Not sure i fully get what you mean chiro
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  4. #4
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    Re: Finding functions of g(x) and f(x)

    g(x) = 3x^2 - 5 f(x)

    g'(x) = (3x^2 - 5 f(x)) '
    g'(x) = (3x^2)' + (-5 f(x)) '
    g'(x) = 3 * 2x + -5 * f ' (x)
    g'(x) = 6x - 5 f ' (x)

    Now set x = 3

    g'(3) = 6*3 - 5 f ' (3)
    g'(3) = 18 - 5 * 5
    g'(3) = 18 - 25
    g'(3) = -7
    Thanks from JellyOnion
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  5. #5
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    Re: Finding functions of g(x) and f(x)

    Thanks mathguy25
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