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Math Help - Logarithmic problem help

  1. #1
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    Logarithmic problem help

    I have to solve x in terms of k

    log x + log (x+9)=k
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  2. #2
    MHF Contributor Also sprach Zarathustra's Avatar
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    log(x) + log(y) = log(xy)
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  3. #3
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    Quote Originally Posted by Also sprach Zarathustra View Post
    log(x) + log(y) = log(xy)


    Sorry wrote that completely wrong, meant to say solve for X in terms of K
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  4. #4
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    Yes, Also Sprach Zarathustra understood that. Apply his hint. If "log(x)+ log(y)= log(xy)" is a general rule, then what is log(x)+ log(x+ 9)?

    Do you know how to solve log(y)= k for y? Do you know what the inverse function to log(x) is?
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  5. #5
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    Quote Originally Posted by HallsofIvy View Post
    Yes, Also Sprach Zarathustra understood that. Apply his hint. If "log(x)+ log(y)= log(xy)" is a general rule, then what is log(x)+ log(x+ 9)?

    Do you know how to solve log(y)= k for y? Do you know what the inverse function to log(x) is?
    I haven't taken pre calc in 3 years so my memory of it is really sketchy..this is a review problem for calc which I'm taking right now.

    Log(x^2+9x)=k
    10^k=(x^2+9x)

    Can't really figure out where to go from there. Like I said I barely remember anything from pre calc since its been about 3 years.
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  6. #6
    Super Member Bacterius's Avatar
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    That's the right way to do it, now notice that it is equal to the equation x^2 + 9x - 10^k = 0, which is a quadratic equation in x with a constant term in k. Therefore you can apply the quadratic formula to isolate x and solve it in terms of k.
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