Does the following integral converge

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In summary, the integral from 0 to 1 given: 1 / [ (x^1/3)(x^2+2x)^1/2 ] dx behaves as 1/x^1/3*x^1/2 = 1/x^1/6 which is convergent as the exponent is <1. However, 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.
  • #1
a1010711
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Homework Statement



the integral from 0 to 1 given:

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

please explain,, thank you!



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
 
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  • #2
a1010711 said:

Homework Statement



the integral from 0 to 1 given:

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

please explain,, thank you!



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
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.
 
  • #3
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
 
  • #4
lanedance said:
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
Substitute WHAT into the integral?
lanedance said:
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
Lanedance, I'm not following you here.
 
  • #5
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
 

1. Does the following integral converge?

This is a common question asked when evaluating integrals. The answer depends on the specific integral and its limits of integration. Some integrals converge, meaning they have a finite value, while others diverge, meaning they do not have a finite value.

2. How do I determine if an integral converges or diverges?

To determine if an integral converges or diverges, you can use various methods such as the comparison test, the limit comparison test, or the integral test. These methods involve evaluating the behavior of the integral as the limits of integration approach infinity.

3. What is the difference between a convergent and a divergent integral?

A convergent integral has a finite value, meaning it converges to a specific number. On the other hand, a divergent integral does not have a finite value and either approaches infinity or oscillates between different values as the limits of integration increase.

4. Can an integral converge for some values of the variable and diverge for others?

Yes, it is possible for an integral to converge for certain values of the variable and diverge for others. This is known as conditional convergence and occurs when the integral has both positive and negative terms, and the sum of these terms is finite for some values of the variable but infinite for others.

5. Is there a general rule for determining if an integral converges or diverges?

No, there is no general rule for determining the convergence or divergence of integrals. The behavior of an integral depends on various factors such as the function being integrated, the limits of integration, and the method used to evaluate the integral. It is important to use different methods to determine the convergence or divergence of integrals and to confirm the results using multiple approaches.

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