Very quick question on logs

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  1. Perpetuallity's Avatar
    • Exalted and Worshipped Member
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    Very quick question on logs
    Rewrite the equation 3^x = 2^{x+1} in the form c^x = d where c and d are constants.

    I've tried numerous things but nothing seems to work. Some hints would be appreciated.
  2. nohomo's Avatar
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    Re: Very quick question on logs
    Note that 2^{x+1} = 2^x\cdot 2
  3. najinaji's Avatar
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    Re: Very quick question on logs
    xlog3 = (x+1)log2

    xlog3 = xlog2 + log2

    xlog3 - xlog2 = log2

    Am I on the right lines?
  4. nohomo's Avatar
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    Re: Very quick question on logs
    I don't think you need logs to answer the question you've asked here OP
  5. Perpetuallity's Avatar
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    Re: Very quick question on logs
    (Original post by nohomo)
    I don't think you need logs to answer the question you've asked here OP
    No, not really, but using natural logs I got it to e^x = 5.54...
  6. shahruk's Avatar
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    Re: Very quick question on logs
    (Original post by Perpetuallity)
    Rewrite the equation 3^x = 2^{x+1} in the form c^x = d where c and d are constants.

    I've tried numerous things but nothing seems to work. Some hints would be appreciated.

    => 3^x = 2 X 2^x

    Dividing both sides by 2^x

    => (3^x)/(2^x) =2

    => (3/2)^x =2

    => 1.5^x = 2

    Is that the answer ?
  7. steve2005's Avatar
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    Re: Very quick question on logs
    (Original post by Perpetuallity)
    No, not really, but using natural logs I got it to e^x = 5.54...
    I think your method is overkill.
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  8. TheGrinningSkull's Avatar
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    Re: Very quick question on logs
    (Original post by najinaji)
    xlog3 = (x+1)log2

    xlog3 = xlog2 + log2

    xlog3 - xlog2 = log2

    Am I on the right lines?
    Then x log(1.5) = log2 (Log rule)

     log((\frac{3}{2})^x) = log2

    (\frac{3}{2})^x = 2
    



Or 1.5^x = 2
    Last edited by TheGrinningSkull; 17-05-2012 at 18:06.
  9. nohomo's Avatar
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    Re: Very quick question on logs
    (Original post by Perpetuallity)
    No, not really, but using natural logs I got it to e^x = 5.54...
    Steve2005 explains a simpler way of answering the question.
  10. steve2005's Avatar
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    Re: Very quick question on logs
    (Original post by Perpetuallity)
    No, not really, but using natural logs I got it to e^x = 5.54...
    Your 5.54 value should be 5,526...BUT no need to involve logs.
  11. Perpetuallity's Avatar
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    Re: Very quick question on logs
    (Original post by steve2005)
    Your 5.54 value should be 5,526...BUT no need to involve logs.
    I know, I truncated (ln3-ln2) in my workings which carried through and changes the final answer slightly.
  12. Pride's Avatar
    • TSR Demigod
    • Location: London
    Re: Very quick question on logs
    (Original post by steve2005)
    I think your method is overkill.
    I couldn't do it that way in an exam situation. I think I just don't know the indices rules to that level.

    Oh wait, yes I do, normally we do it the other way round, multiply out the brackets, never tried that. nice...
    Last edited by Pride; 17-05-2012 at 18:36.
  13. steve2005's Avatar
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    Re: Very quick question on logs
    (Original post by Perpetuallity)
    I know, I truncated (ln3-ln2) in my workings which carried through and changes the final answer slightly.
    I realise that is what you did BUT it is not correct to round as you progress through a calculation.

    ROUNDING should only occur at the final answer stage.
  14. najinaji's Avatar
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    Re: Very quick question on logs
    (Original post by TheGrinningSkull)
    Then x log(1.5) = log2 (Log rule)

     log((\frac{3}{2})^x) = log2

    (\frac{3}{2})^x = 2
    



Or 1.5^x = 2
    Excellent! Hoped that would be the right method. :yy:
  15. Perpetuallity's Avatar
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    Re: Very quick question on logs
    (Original post by steve2005)
    I realise that is what you did BUT it is not correct to round as you progress through a calculation.

    ROUNDING should only occur at the final answer stage.
    I'll use the "ans" button on the calculator next time. Thanks for the help.
  16. TheGrinningSkull's Avatar
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    Re: Very quick question on logs
    (Original post by najinaji)
    Excellent! Hoped that would be the right method. :yy:
    Yea, it looks right, it makes sense as well, and then you can solve for x
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