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Homework Help: Does the following integral converge

  1. Feb 25, 2009 #1
    1. The problem statement, all variables and given/known data

    the integral from 0 to 1 given:

    1 / [ (x^1/3)(x^2+2x)^1/2 ] dx

    plz explain,, thank you!



    3. The attempt at a solution

    At 0 the integrand behaves as 1/x^1/3*x^1/2 = 1/x^1/6 which is convergent as the exponent is <1
     
  2. jcsd
  3. Feb 26, 2009 #2

    Mark44

    Staff: Mentor

    The integrand does not behave as 1/[x^(1/3)*x^(1/2)]. Even if it did, 1/x^(1/6) becomes infinitely large as x approaches zero. You're probably thinking of the behavior of a p series, where n is growing infinitely large.

    My guess is that this is a divergent integral because of what's happening close to zero. The integrand is less than 1/x^(4/3), which is a function I can integrate and for which the definite integral diverges. Unfortunately, our integrand is less than a function whose antiderivative diverges, so that's no help.

    On the other hand, x^(1/3)*sqrt(x^2 + 2x + 1) > x^(1/3)*sqrt(x^2 + 2x), so
    1/[x^(1/3)*sqrt(x^2 + 2x + 1)] < 1/[x^(1/3)*sqrt(x^2 + 2x)] ,
    which means that
    1/[x^(1/3)*(x + 1)] < 1/[x^(1/3)*sqrt(x^2 + 2x)]

    If I can show that the antiderivative of the function on the left above is divergent, that means that the one on the right is, too. Unfortunately, now, I can't come up with an antiderivative for the function on the left.
     
  4. Feb 26, 2009 #3

    lanedance

    User Avatar
    Homework Helper

    could maybe do it 2 ways based on the integral 1->inf of 1/x^n converging iff n>1

    2 ways potentially

    substitute straight into your integral, changing integration from 0to1 to 1 to infinty, and then use some comparison theorems

    or substitute into the integral 1->inf of 1/x^n which will change it to 0 to 1, should give you a condition based on n for this integral to converge

    i'm thinking it probably does converge at this stage
     
  5. Feb 26, 2009 #4

    Mark44

    Staff: Mentor

    Substitute WHAT into the integral?
    Lanedance, I'm not following you here.
     
  6. Feb 26, 2009 #5

    lanedance

    User Avatar
    Homework Helper

    woops sorry missed out the key part... editing before submitting then not reading properly

    meant to say substitute u = 1/x, into either integral to change to zero limit to a limit at infinity, or vice versa

    so du = -1/x^2 dx ---> -du/u^2 = x
     
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