Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help for my daughter

  1. Mar 5, 2008 #1

    Can anyone please help me re integration from 0 to infinity of the following;

    (e^0.03t)((50e^0.07t-10)/k)^-0.5 dt

    Yours sincerely,

  2. jcsd
  3. Mar 5, 2008 #2

    Gib Z

    User Avatar
    Homework Helper

    Doing that problem analytically is probably not possible, but if we very vaguely approximate the integrand, we can see what it is for most practical purposes.

    Your integral is simplifies to [tex]\sqrt{\frac{k}{50}} \int^{\infty}_0 \frac{e^{0.03t}}{e^{0.07t} - 0.2} dt[/tex].

    The next step takes away a lot of accuracy, but its the best i can really think of that doesnt take forever to do. We pretend we don't see the -0.2 in the denominator lol.

    If we do that and evaluate the integral, we should get [tex]5\sqrt{ \frac{k}{2} }[/tex].
  4. Mar 5, 2008 #3


    User Avatar
    Science Advisor

    What class is your daughter in that she would have a problem like that?
  5. Mar 5, 2008 #4
    Gib Z,
    Thank you. I have sent this to my daughter.

    My daughter is doing economics in university and this has something to do with the reserves of oil in the world.
  6. Mar 5, 2008 #5


    User Avatar
    Science Advisor
    Homework Helper

    Using maple the integral

    [tex]\int^{\infty}_0 \frac{exp(0.03t)}{exp(0.07t) - 0.2}dt[/tex]

    is approximately 27.07839303
  7. Mar 6, 2008 #6

    Gib Z

    User Avatar
    Homework Helper

    O thats pretty good then =] When we ignored that -0.2, it was 25.
  8. Mar 6, 2008 #7
    Am I missing something, or shoudn't the integral portion be

    [tex]\int^{\infty}_0 \frac{e^{0.03t}}{\sqrt{e^{0.07t} - 10}}dt[/tex]
  9. Mar 6, 2008 #8
    Yes Theo, having spoken to my daughter I think you are correct. What would the answer be then? All replies are very welcome.
  10. Mar 6, 2008 #9


    User Avatar
    Science Advisor
    Homework Helper

    According to maple, the integral


    is approximately 28.48936147
  11. Mar 7, 2008 #10

    Gib Z

    User Avatar
    Homework Helper

    Sorry my bad >.<" Maybe thats why I was so surprised that my 'approximation' was quite close, because I only ignored 0.2 instead of a 10. If we try to ignoring thing again I'm sure it would be quite a bit off.
  12. Mar 7, 2008 #11
    I'm still looking at this, but I'm not done. However, I don't think the value is that high. Initially, I'm getting something roughly half this value.

    more to come.
  13. Mar 7, 2008 #12
    After further review, I indeed get results that agree with nicksauce, ie, The integral is approx equal to 28.489361...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook