ChetIs the Integral Convergent?

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Homework Help Overview

The discussion revolves around determining the convergence of the integral of the function 1/sqrt(x^4+x^2+1) from 1 to infinity. Participants are exploring methods to evaluate the integral and discussing the implications of certain transformations on its convergence.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss comparing the original function to a simpler function to assess convergence. There are attempts to complete the square and transform the integral into a more manageable form. Some participants suggest using trigonometric substitution as a potential method for finding the antiderivative.

Discussion Status

There is an ongoing exploration of different approaches to evaluate the integral. Some participants have provided guidance on transformations and substitutions, while others are questioning the necessity of integrating the exact expression. The discussion reflects a mix of interpretations and methods being considered without a clear consensus on the best approach.

Contextual Notes

Participants are working within the constraints of homework rules, focusing on understanding the convergence of the integral rather than finding a complete solution. There is mention of specific terms in the integral that may affect convergence, which are under discussion.

Neil21
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Homework Statement


Find whether the integral is convergent or not, and evaluate if convergent.

Homework Equations


integral 1/sqrt(x^4+x^2+1) from 1 to infinity

The Attempt at a Solution


1/sqrt(x^4+x^2+1)<1/sqrt(x^4)
1/sqrt(x^4)=1/x^2 which is convergent for 1 to infinity and is 1
therefore, the original function is convergent
I can't seem to find the antiderivative of the original function after this.
 
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Try completing the square
 
Ok, so I complete the square and got \int \frac{1}{((x^{2}+\frac{1}{2})^{2}+\frac{3}{4})^{\frac{1}{2}}} . I am still not seeing a way find an antiderivative though.
 
Neil21 said:
Ok, so I complete the square and got \int \frac{1}{((x^{2}+\frac{1}{2})^{2}+\frac{3}{4})^{\frac{1}{2}}} . I am still not seeing a way find an antiderivative though.
Don't forget the dx.
$$\int \frac{dx}{((x^{2}+\frac{1}{2})^{2}+\frac{3}{4})^{\frac{1}{2}}}$$

I haven't worked this through, yet, but here's what I would do, FWIW.
Let u = x2 + 1/2
So ##x = \sqrt{u - 1/2}##
Then du = 2xdx
So ##dx = \frac{du}{2x} = \frac{du}{2\sqrt{u - 1/2}}##

After making the substitution and getting everything in terms of u and du, I would try for a trig substitution. I think that might work.
 
You don't need to integrate the exact expression. If the 3/4 wasn't in there, would the integral be convergent?

Chet
 
Last edited:
Trig substitution is always my first instinct when I see a fraction with higher powers of x.
 
Chestermiller said:
You don't need to integrate the exact expression. If the 3/4 wasn't in there, would the integral be convergent?

Chet
@chet, I think he already established that in the opening post.
 
Mark44 said:
Don't forget the dx.
$$\int \frac{dx}{((x^{2}+\frac{1}{2})^{2}+\frac{3}{4})^{\frac{1}{2}}}$$

I haven't worked this through, yet, but here's what I would do, FWIW.
Let u = x2 + 1/2
So ##x = \sqrt{u - 1/2}##
Then du = 2xdx
So ##dx = \frac{du}{2x} = \frac{du}{2\sqrt{u - 1/2}}##

After making the substitution and getting everything in terms of u and du, I would try for a trig substitution. I think that might work.
What I first saw was rewriting the expression as $${\frac{1}{(x^4+x^2+1)^{1/2}}}={\frac{1}{((x^2+({\frac{1}{2}))^2}+(\frac{\sqrt{3}}{2})^2)^{1/2}}}$$.
Which I'm pretty sure can be solved with a trig sub.
 
Mark44 said:
@chet, I think he already established that in the opening post.
Ooops. Missed that. Sorry guys.

Chet
 

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