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Homework Help: Proving that two Ln Equations equal each other

  1. Jun 6, 2010 #1
    1. The problem statement, all variables and given/known data

    Differentiates to: 0.01 ln8 x 8^0.01x
    Using derive on a graphics calculator, the answer given is:
    3x2^3x-200/100 ln2/25
    Prove that the two equal each other using index and logarithm laws.

    2. Relevant equations

    3. The attempt at a solution

    Ah, well, I've had plenty of attempts. I've no idea if any are on the right track, but I'm working backwards from the second equation and have tried to split the equation into three parts to simplify. Needless to say, it didn't work. I've also tried simplifying the top of the equation to no avail. Any help would be appreciated.
  2. jcsd
  3. Jun 6, 2010 #2


    Staff: Mentor

    Please use parentheses on the expression above, as it is very ambiguous. Also, do not use 'x' to indicate multiplication since x is a variable in this expression.
  4. Jun 7, 2010 #3
    Last edited by a moderator: May 4, 2017
  5. Jun 7, 2010 #4
    What happened to the 2^(-200/100)? What is 3*ln(2)?
  6. Jun 7, 2010 #5
    I'm probably wrong, but I went:
    2(3x/100) - 2
  7. Jun 7, 2010 #6
    But in fact 2(3x/100)-2 = 2(3x/100)/22.
  8. Jun 7, 2010 #7
    I'm not sure, I just need to prove that the two equations are equal. :|
  9. Jun 7, 2010 #8
    Yes, if you look carefully, you'll see that your original simplification was wrong, because it forgot the 22 in the denominator. After you fix that, you can simplify further by considering how to condense 3*ln(2).
  10. Jun 7, 2010 #9
    I'm sorry for sounding dumb, but could you please explain why 22 needs be in the denominator? Does not seem to be a good maths day today.
  11. Jun 7, 2010 #10
    Before you went from 2(3x/100) - 2 to 2(3x)0.01. These two are not equal, because in the first you are subtracting 2 from the exponent. In other words, the second expression is 22 times bigger than the first.
  12. Jun 8, 2010 #11
    Hmm, okay then. That did occur to me initially, but I overlooked it because it started to match the other equation. How would you solve it? I've tried a few times to no avail.
  13. Jun 8, 2010 #12
    Changing to 80.01x is fine, as long as you remember to divide the whole expression by 22. This should get you closer. Next, simplify 3*ln(2).
  14. Jun 8, 2010 #13


    Staff: Mentor

    The result from the calculator was
    [tex]\frac{3 ln 2 * 2^{(3x - 200)/100}}{25}[/tex]
    [tex]= \frac{ln(2^3) * 2 ^{.03x - 2}}{25}[/tex]
    [tex]= \frac{ln(8) * 2^{.03x} * 2^{-2}}{25}[/tex]

    I used the fact that ab - c = ab * a-c.
    Another property of exponents that will be useful for finishing this is abc = (ab)c.
  15. Jun 8, 2010 #14
    Ah, I solved it! Thank you very much.
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