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Prove that two definite integrals are equal

  1. Sep 23, 2010 #1

    Char. Limit

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    Gold Member

    1. The problem statement, all variables and given/known data
    I want to prove the following statement:

    [tex]\int_0^{ln(2)} \sqrt{e^{2t} + 4 e^{4t} + 9 e^{6t}} dt = \int_1^2 \sqrt{1 + 4 t^2 + 9 t^4} dt[/tex]


    2. Relevant equations



    3. The attempt at a solution

    To be honest, I'm not sure how to do this. I tried a substitution [tex]t=e^t[/tex] for the second integral, but it didn't really go all that well. How do you do this, assuming that direct calculation is not allowed?
     
  2. jcsd
  3. Sep 23, 2010 #2

    rl.bhat

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    Homework Helper

    Let e^t = x.
    Then e^t*dt = dx,

    When t = 0, x = 1 and when t = ln(2), x = 2.
    Substitute these values in the given integration.
    Then replace x by t, because the definite integration does not depend on the variables whether it is x or t or any other.
     
  4. Sep 23, 2010 #3
    Factor out [tex]e^{2t}[/tex] of the first integral and make t= e^t and change the bounds of integration.
     
  5. Sep 23, 2010 #4
    I seem to remember if you have two Riemann sums R1 and R2 if they converge to the same point then they are equal. Could this be used here?
     
    Last edited: Sep 23, 2010
  6. Sep 23, 2010 #5

    Mark44

    Staff: Mentor

    It's less confusing to use a different variable; say, u = e^t.
     
  7. Sep 23, 2010 #6

    Mark44

    Staff: Mentor

    A better approach is to follow the advice given by rl.bhat and ╔(σ_σ)╝.
     
  8. Sep 23, 2010 #7
    Why is that? Cause if the Riemann sums Converge to the same number then integrals are equal aren't they? Maybe I'm just a silly girl.
     
  9. Sep 23, 2010 #8
    First of all you have to write out the riemann sums and show it converges to something and write out down the something.

    Then you have to prove the two riemann sums are equal.
     
  10. Sep 23, 2010 #9
    Well then I wasn't far of was I ? ;)
     
  11. Sep 23, 2010 #10
    Well your suggestion was not "wrong". However, it will not be helpful to the OP as the technicality involving your suggestion require a lot of work. And remember the riemann sum ,by definition, is not something you can easily manipulate. When you have functions that are not polynomials , integrating from the definition becomes non trivial even for relatively "simple" functions.

    How about you try out your suggestion and see if it works out well.:-)
     
  12. Sep 23, 2010 #11
    If I do I will properly get an infraction...
     
  13. Sep 23, 2010 #12
    You don't have to post it here and even if you do, you would not get an infraction. Just confess if you cannot do it.
     
  14. Sep 23, 2010 #13
    I posted something simular last year and got an infraction so. But yes I could do it...
     
  15. Sep 23, 2010 #14
    Do it and send it as a private message to me :-).
    Perhaps you have some clever tricks that are worth seeing.
     
  16. Sep 23, 2010 #15
    If you can solve the differential in my other post, then we have a deal.
     
  17. Sep 23, 2010 #16
    If you mean your non linear differential equation then, no. I do not know how to solve it. I haven't taking any courses on non linear differential equations neither am I interested in learning how to do so, atm.
     
  18. Sep 23, 2010 #17
    If you mean your non linear differential equation then, no. I do not know how to solve it. I haven't taking any courses on non linear differential equations neither am I interested in learning how to do so, atm.
    I know my limitations.:-)
     
  19. Sep 23, 2010 #18
    yeah it was this one here

    https://www.physicsforums.com/showthread.php?t=431007

    OKay fair enough..
     
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