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Natural log problem

  1. Jul 25, 2007 #1
    Problem: If [tex]2^{x}=3[/tex], what does 3^x equal?

    I solved this by taking the [tex]log_{2}[/tex] of both sides. However, the book's solution involves taking the natural log of both sides. Can someone show me why that would work? I don't get what the x power changes to a x term. The natural log is base 10 right?
     
    Last edited: Jul 25, 2007
  2. jcsd
  3. Jul 25, 2007 #2

    Kurdt

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    What exactly are you asking? If 23, what..... means nothing.

    Do you mean:

    [itex] 2^3 = x [/itex] then what does [itex]3^x[/itex] equal?
     
  4. Jul 25, 2007 #3
    My bad... I've edited the problem
     
  5. Jul 25, 2007 #4
    Also, someone please answer this question. How come: [tex]\sqrt{(-2)^{2}}=2[/tex]??

    This is very mind boogling because (-2)^2=4 and the square root of 4 is 2... but, why is it wrong to simplify just by canceling out the power of 2 with the square root? I've always done it that way... =/
     
    Last edited: Jul 25, 2007
  6. Jul 25, 2007 #5

    Kurdt

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    Ok no probs.

    Because exponentials are defined in terms of the exponential function (hence the name) and the natural logarithm is the inverse to the exponential function, it is easy to write a logarithm to the base of any number in terms of natural logs. For example, we define [itex]\log_a (x)=y[/itex] to be the inverse of [itex] x= a^y[/itex]. But from our knowledge of exponentials we can say:

    [tex]x=a^y=e^{y\ln (a)} \rightarrow \ln(x)=y\ln(a) \rightarrow y=\frac{\ln (x)}{\ln (a)} [/tex]
     
  7. Jul 25, 2007 #6
    The mistake that many people make (especially those in physics, because math majors look out for such mistakes) is that [tex]\sqrt{x^2}=x[/tex]. That is wrong. For example, if [tex]x=(-2)^2[/tex] then [tex]\sqrt{(-2)^2}=2[/tex].

    The correct version states that,
    [tex]\sqrt{x^2}=|x|[/tex].
     
  8. Jul 25, 2007 #7

    So [tex]ln[/tex] is basically equal to [tex]log_{e}[/tex] right? What is [tex]e^{log_{e}}[/tex]??
     
  9. Jul 25, 2007 #8
    Is there any reason why you can't just take cancel the sqrt. and power of 2? It is different for cube roots and cube powers right?

    So the 4th root of (-4)^4 is equal to 4 right?
     
  10. Jul 25, 2007 #9

    Kurdt

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    Yes, ln is a shorthand for natural log or loge. The natural logarithm is the inverse function of the exponential function and thus:

    [tex] e^{\ln a} = a[/tex]
     
    Last edited: Jul 25, 2007
  11. Jul 25, 2007 #10

    Kurdt

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    Sorry, I didn't see in your original post that you asked if natural log was log to the base ten. Natural log is log to the base e.
     
  12. Jul 25, 2007 #11
    What Kummer said was correct about the absolute value of two. You can't just cancel it out, you may cancel it out though if it is (square root of x)^2 as long as the radical symbol is inside the parentheses then you may cancel out.
     
  13. Jul 26, 2007 #12

    HallsofIvy

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    Getting back to the first question:
    If 2x= 3, then log(2x)= x log(2)= log(3) so
    [tex]x= \frac{log(3)}{log(2)}[/itex]

    Now, 3x= 3log(3)/log(2).

    That isn't going to be any simple number (it's about 5.7).
     
  14. Jul 26, 2007 #13

    VietDao29

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    Yup, exactly. Well, since y = x3 is a 1-to-1 function, i.e, for every [tex]x_0 \neq x_1[/tex], we'll obtain: [tex]y_0 = x_0 ^ 3 \neq y_1 = x_1 ^ 3[/tex], so given one y, we can find only one x such that y = x3.

    But y = 2 is a different thing. 2 numbers a, and -a, are different (for a <> 0), but when squared, they'll become: (a2) = a2, and (-a)2 = (-1)2 a2 = a2, so, we have: [tex](a) ^ 2 = (-a) ^ 2 = a ^ 2[/tex]. So, for any y (not 0), there'll be 2 different x's, one positive, one negative, such that, when squared, they'll give y.

    Say, y = 4, then 2 x's are 2, and -2.

    Square root function will only return the non-negative value.

    Say, we have: y = 4, so sqrt(y) = 2; k = 16 ~~> sqrt(k) = 4.

    So, in general, we have: [tex]\sqrt{a ^ 2} = |a|[/tex]

    To take the negative value, we can assign the minus sign in front of it, like this:

    [tex]t = 36 \Rightarrow - \sqrt{t} = -6[/tex]

    -------------------------

    And it's the same for even-th root, like: [tex]\sqrt[4]{x ^ 4} = |x| , \ \sqrt[6]{x ^ 6} = |x|, ...[/tex], blah, blah, blah, so on. :)

    Hope that's clear. :)
     
    Last edited: Jul 26, 2007
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