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Homework Help: Help with logarithms

  1. Jan 3, 2010 #1
    I have [tex]2lnx = xln2[/tex]
    where [tex]x\ne2[/tex]

    if you start by dividing both sides by ln2
    is the following legal?

    [tex]\frac{2lnx}{ln2} \rightarrow x = 2ln(x-2)[/tex]

    [tex]e^{2ln(x-2)} = (x-2)^2[/tex]

    [tex]x = (x-2)^2 \implies x = 4[/tex]
  2. jcsd
  3. Jan 3, 2010 #2


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    I'm confused at what you've done.

    Did you manipulate [tex]\frac{2lnx}{ln2}[/tex] and turn it into [tex]2ln(x-2)[/tex]? Because this is generally not correct.

    From what I can gather, you've just fluked your way into breaking a rule but still finding a solution for x. That equation isn't simple to solve.
  4. Jan 3, 2010 #3

    [tex]\frac{ln~x}{ln~y} \implies ln(x-y)[/tex]

  5. Jan 3, 2010 #4

    [tex]2lnx = xln2\Rightarrow \frac{d[2lnx]}{dx}=ln2\frac{dx}{dx}\Rightarrow \frac{2}{x}=ln2\Rightarrow x=\frac{2}{ln2}[/tex]
    Last edited: Jan 3, 2010
  6. Jan 3, 2010 #5
    Hm, no!
  7. Jan 3, 2010 #6
    Then tell me what did I do wrong. You follow step by step what I did?
    Last edited: Jan 3, 2010
  8. Jan 3, 2010 #7
    No, you have it completely backwards... :smile:
    [tex]\ln\left(\frac{x}{y}\right) = \ln x - \ln y[/tex]

    but this isn't going to help actually solve the equation.
  9. Jan 3, 2010 #8


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    Hint: review the rules for manipulating logs. One of the says that
    A ln(B) = ____?​
  10. Jan 3, 2010 #9
    I got fool too!!!! And I destroy all the evidence already!!!:rofl::tongue:

    I did it differently and I still don't see why he claimed my answer in #4 was wrong!!!
  11. Jan 3, 2010 #10


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    He claimed it was wrong because it is wrong...

    You took the derivative and solved for that. Yeah you found the turning point, but not a root of the original equation.
  12. Jan 3, 2010 #11


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    You seem to be thinking that d[2lnx]/dx = 2lnx. It's not. It's equal to 2/x.
  13. Jan 3, 2010 #12


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    No no, yungman never asserted that. What he did was correct, but only for finding the turning point of [itex]y=2lnx-xln2[/itex].

    It's kind of confusing with all the arrows and equals signs, so I'll clear it up:

    [tex]2lnx = xln2[/tex]




    But again, it doesn't help finding the solution x=4 to this problem (which btw, the OP found by an illegal move)
  14. Jan 3, 2010 #13
    if the problem is just as simple as solving the equation for x then just raise everything to the e.

    e^(lnx)^2=e^(ln2)^x this trivializes and is simple to solve.
  15. Jan 3, 2010 #14


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    Yes so you have [itex]x^2=2^x[/itex] but really, this isn't as simple to solve as you claim. It's just that we are lucky enough to have nice integer solutions for x>0. If we ignored the domain on the original equation and tried to instead solve this one, you can only get a numerical approximation for the solution in [itex]-1<x<0[/itex]
  16. Jan 3, 2010 #15
    You loss me about "turning point"!!! What is a turning point?

    Who said that the answer is x=4? James concluded x=4 with a wrong assumption that [tex]\frac{lnx}{ln2}=ln(x-2)[/tex].

    I conclude [tex]x=\frac{2}{ln2}[/tex].....which is a constant number.
  17. Jan 3, 2010 #16
    Yes I tried that and didn't go no where!!!:rofl:
  18. Jan 4, 2010 #17


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    You know about derivatives but not turning points?

    But the answer is x=4. Just try it (and btw, 4 is a constant number too :tongue:)




    So x=4 is a solution. And x=2 is too, but not 2/ln2.
  19. Jan 4, 2010 #18
    I self study most of my calculus!!! Things sounds common to you might not be to me!!!! The only other class I took was ODE and I have people laughing at me on some stuff too!!:tongue2: But I was the first in class for the semester.

    Do you mean the point that the function turn from a convex to a concave? I remember I read something about it a few years ago. But still never heard of turning point!!!

    I still don't get why my method don't produce the answer. I know I got that by taking the derivative on both side, I have seen work problems using this method. Do I have to keep bitting my nails and wait until James come up with the answer??
  20. Jan 4, 2010 #19


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    It's because [tex]f(x)=0[/tex] and [tex]f'(x)=0[/tex] generally have different solutions. When you differentiated, you got a new equation, and its solution is not a solution to the original equation.
  21. Jan 4, 2010 #20
    Thanks, I got it.

    This is where I am at:

    [tex]xln2=2lnx\Rightarrow x=\frac{2}{ln2}x[/tex]

    [tex]\Rightarrow \frac{x}{lnx}=\frac{2}{ln2}[/tex]

    It is easy to see x=2. I still don't see x=4.
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