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Math Help - Logarthim/Exponential help..

  1. #1
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    Logarthim/Exponential help..

    Very quick one this:

     <br />
3ln(x+7) = ln(x+7)^3<br />

    I am sure the above is correct, but does:

     <br />
exp^ (ln(x+7)^3) = (x+7)^3 or (x+7)<br />

    (Please excuse the wrong sized bracket after the exp function - I can't seem to get it to go the right size in LaTeX)

    I am assuming it's (x+7)^3 but have been given x+7 as an answer from an online calculator.

    Please help - I'm rubbish at logarithms and need to improve asap due to a home study course coming up!
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  2. #2
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    e^{\ln{f(x)}} = f(x) and e^{g(x)\cdot \ln{f(x)}} = e^{\ln{f(x)}^{g(x)}} = f(x)^{g(x)}.
    In your case we have e^{3\ln(x+7)} = e^{\ln(x+7)^3} = (x+7)^3.
    So you're correct; the online calculator is wrong.
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  3. #3
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    O.K - I was hoping it would have been x+7 though,

    Now I've got a really difficult equation to solve...
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  4. #4
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    Quote Originally Posted by MaverickUK82 View Post
    O.K - I was hoping it would have been x+7 though,
    You're not at all pleased about being correct then?
    Good luck with solving the big equation you've got.
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  5. #5
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    No - I am pleased with being correct.

    Its just now i have a long equation to simplify - how would you go about the following:
     <br />
ln(x+2)^2 + ln(x+9)^-^2 + ln(x^2-1)^2<br />

    Somethign like:

     <br />
ln((x+2)^2(x+9)^-^2(x^2-1)^2)<br />

    Because it doesn't look simpler to me!
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  6. #6
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    Then go the other way: 2ln(x+ 2)- 2ln(x+ 9)+ 2ln(x-1)+ 2ln(x+ 1). Does that look simpler to you?
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  7. #7
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    Halls of Ivy, that's what I started with originally:

    2ln(x+2)-2ln(x+9)+3ln(x^2-1)

    but simplified it to:

    ln(x+2)^2 + ln(x+9)^-^2+ln(x^2-1)^3

    The post before this was slightly incorrect (exponent of 2 not 3).
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  8. #8
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    Consider the two possibilities, let this expression be equal to y

    e^y = e^{\ln[(x+2)^2(x+9)^-^2(x^2-1)^3]} OR e^y = e^{ln(x+2)^2 + ln(x+9)^-^2+ln(x^2-1)^3}

    which of those looks easier to simplify?
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  9. #9
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    The actual full question is:

    solve for y and simplfy as far as possible.

    3ln(y+4)=ln(x+2)^2 + ln(x+9)^-^2+ln(x^2-1)^3

    I can separate y on the left by changing 3ln(y+4) to ln(y+4)^3, using the exponential rule and taking cube root and moving over the 4, but it's the simplification I'm struggling with.
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  10. #10
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    Quote Originally Posted by e^(i*pi) View Post
    Consider the two possibilities, let this expression be equal to y

    e^y = e^{\ln[(x+2)^2(x+9)^-^2(x^2-1)^3]} OR e^y = e^{ln(x+2)^2 + ln(x+9)^-^2+ln(x^2-1)^3}

    which of those looks easier to simplify?
    The first: e^y = e^{\ln[(x+2)^2(x+9)^-^2(x^2-1)^3]}=(x+2)^2(x+9)^-^2(x^2-1)^3 ?
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  11. #11
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    Which is the same as:

    \frac{(x+2)^2(x^2-1)^3}{(x+9)^2}

    ?
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  12. #12
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    Quote Originally Posted by MaverickUK82 View Post
    The actual full question is:

    solve for y and simplfy as far as possible.

    3ln(y+4)=ln(x+2)^2 + ln(x+9)^-^2+ln(x^2-1)^3

    I can separate y on the left by changing 3ln(y+4) to ln(y+4)^3, using the exponential rule and taking cube root and moving over the 4, but it's the simplification I'm struggling with.
    You'd be better off clearing the log terms first


    Use what you did in post 4

    \ln(y+4)^3 = \ln[(x+2)^2(x+9)^-^2(x^2-1)^2]

    Those logarithms will cancel to give (y+4)^3 = \dfrac{(x+2)^2(x^2-1)^2}{(x+9)^3}

    You can take the cube root now and remember that \sqrt[3]{ab} = \sqrt[3]{a} \cdot \sqrt[3]{b}

    =================================

    If you want to use the form in your last post you will get

    ln(y+4)^3=ln(x+2)^2 + ln(x+9)^-^2+ln(x^2-1)^3

    e^{ln(y+4)^3}=e^{ln(x+2)^2 + ln(x+9)^-^2+ln(x^2-1)^3}

    You can use the laws of exponents to simplify the RHS

    (y+4)^3 = e^{\ln(x+2)^2} \cdot e^{\ln(x+9)^{-2}} + e^{\ln(x^2-1)^3} = (x+2)^2 \cdot (x+9)^{-2} \cdot (x^2-1)^3
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  13. #13
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    Quote Originally Posted by MaverickUK82 View Post
    Which is the same as:

    \frac{(x+2)^2(x^2-1)^3}{(x+9)^2}

    ?
    Yes
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  14. #14
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    Ah - fantastic - so I was on the right tracks - tricky little things - logarithms.

    What did you mean whne you said "Those logarithms will cancel to give" I have not heard that term given with regards to logarithms.
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  15. #15
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    Quote Originally Posted by MaverickUK82 View Post
    Ah - fantastic - so I was on the right tracks - tricky little things - logarithms.

    What did you mean whne you said "Those logarithms will cancel to give" I have not heard that term given with regards to logarithms.
    I just meant that because the base of the logarithm is the same (base e) so too must the expressions inside them be the same. Cancels is probably pretty dodgy terminology for logs lol
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