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Homework Help: Rewriting a log

  1. Jun 29, 2008 #1
    1. The problem statement, all variables and given/known data

    Use the Laws of logarithms to rewrite the expression in a form with no logarithm of a product, quotient or power.
    ln (x^10*sqrt((y^19)/(z^7))) = a ln(x)+b ln(y)+c ln(z)


    2. Relevant equations

    ln (x^a)=a ln(x)

    3. The attempt at a solution

    I know that it will start =10 ln(x), but I don't know what the square root implies. To specify, if I had the equation ln(sqrt(xy)), I thought the answer would be ln(x)+2 ln(y), but this isn't the case. Can anyone explain what happens to the square root when rewriting this expression? Thanks for your help!
     
  2. jcsd
  3. Jun 29, 2008 #2

    chroot

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    Taking the square root of a quantity is the same as raising that quantity to the 1/2th power. In other words, the square root is interchangeably just an exponent of 1/2.

    - Warren
     
  4. Jun 30, 2008 #3

    HallsofIvy

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    Another very relevant equation for this problem is ln(ab)= ln(a)+ ln(b).

    As chroot told you sqrt is "1/2" power. Notice that even if you had 2nd power, you would NOT have ln((xy)2)= ln(x)+ 2 ln(y). ln((xy)[2= 2ln(xy)= 2[ln(x)+ ln(y)]= 2ln(x)+ 2ln(y).

    Now, ln(sqrt(xy))= ln((xy)1/2)= what?
     
  5. Jun 30, 2008 #4
    thanks for your help, both of you, I was able to figure out the problem with this!
    Halls of Ivy,
    ln(sqrt(xy))= ln((xy)1/2)= 1/2ln(xy)= 1/2ln(x)+ 1/2ln(y)
     
  6. Jun 30, 2008 #5

    HallsofIvy

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    Exactly!
     
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