Solving 2 Tricky Math Problems: Bounded Area & Convergence of Sequence

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Discussion Overview

The discussion revolves around two mathematical problems: determining whether the area bounded by a specific curve in the first quadrant is finite, and assessing the convergence of a given sequence. Participants explore integration techniques and convergence tests, seeking hints and clarifications on their approaches.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions whether the area under the curve \( y= \frac{x} {2*(x^2+2)^{7/8}} \) is finite and expresses difficulty in integrating it.
  • Another suggests using the comparison test with a simpler function to determine convergence.
  • A participant proposes a substitution method for integration, indicating that the integral can be computed.
  • There is a discussion about the relationship between the convergence of the integral and the finiteness of the area, with some asserting that if the integral converges, the area is finite.
  • Another participant expresses confusion regarding the hints provided for the sequence convergence, particularly about the application of the comparison limit test.
  • One participant claims to have found that the area is finite and that the sequence converges to 0, citing limits and comparisons to known functions.
  • Concerns are raised about the bounds of integration and the implications of the integral's limit, with some participants clarifying that the area cannot be zero if the function is positive.
  • There is a challenge regarding the correctness of the integral's convergence, with a request for clarification on the bounds used in the integration.

Areas of Agreement / Disagreement

Participants express differing views on the convergence of the integral and the implications for the area. While some assert that the area is finite based on their calculations, others question the validity of those calculations and the bounds used, indicating that the discussion remains unresolved.

Contextual Notes

Participants have not fully agreed on the correct bounds for integration or the interpretation of the results, leading to uncertainty about the conclusions drawn regarding the area and the sequence convergence.

yeuVi
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There are two problems I got stuck...

1. Is the area in the first quadrant bounded between the x-axis and the curve

[tex]\ y= \frac{x} {2*(x^2+2)^{7/8}}[/tex]

finite? This one, I used the Area formula... but then I cannot integrate it... and then how to determine if it's finite or not?

2. Does the sequence

[tex]\ an= \frac{cos(n)+1}{n^3}[/tex]

converge?

How do you do this one? I need some hints to find the limit...

Thank a lot guys ^_^
 
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1) Try using the comparison test with a simpler function that you can integrate (or know about it's convergence).

2)Hint: -1<=cos(n)<=1
 
You can integrate this

[tex]\frac{1}{2}\int \frac{x}{\left(x^{2}+2\right)^{\frac{7}{8}}} \ dx[/tex]

Make the substitution

[tex]x^{2}+2=u[/tex]

Daniel.
 
But the first one, after I get the integrate, what should I do next? I mean, it's is finite if the integrate limit is converge?

The secon one, I do not really understand the hint... can u explain a little more detail? I guess you want me to apply the comparion limit test for a sequence? I though that test is only for series...

Thanx every1 for helping me... ^_^
 
yeuVi said:
But the first one, after I get the integrate, what should I do next? I mean, it's is finite if the integrate limit is converge?

Yes, if the integral converges you'd say the area was finite. If the integral diverges then you'd say the area was "infinite". (note since the integeand is positive there can only be one kind of divergence, growing without bound).

yeuVi said:
The secon one, I do not really understand the hint... can u explain a little more detail? I guess you want me to apply the comparion limit test for a sequence? I though that test is only for series...

[tex]0\leq a_n \leq \frac{2}{n^3}[/tex]. Think squeeze theorem.
 
Thanx a lot. I tried ur hints and got some results. Can u guys check if they correct?

The 1st one, I got the area is finite...

The 2nd one, the sequence is convergence, since

[tex]\ \lim{0} = \lim{ \frac{2}{n^3}} =0[/tex]

so [tex]\ \lim{a_n}=0[/tex]

therefore the sequence is converge to 0...
 
yeuVi said:
The 1st one, I got the area is finite...

How about posting the details of your work?

yeuVi said:
The 2nd one, ... therefore the sequence is converge to 0...

Looks good.
 
The 1st one, I integrated it by substitution u= x^2+2

so the integration is

1/4 * 8* (u^(1/8))

It's converge to 0, so the area 's finite...
 
The function is always positive when x>0, so how could the area be 0?

HINT: What bounds did you integrate over? Remember, this is an area, so you should be taking a definite integral!
 
  • #10
Oh, it's not the area is 0, but the limit of the integral is 0...
 
  • #11
what limit of the integral?
 
  • #12
shmoe said:
Yes, if the integral converges you'd say the area was finite. If the integral diverges then you'd say the area was "infinite". (note since the integeand is positive there can only be one kind of divergence, growing without bound).

Oh, I was following his hint...
 
  • #13
The hint is fine, but I don't see how you found the integral to be convergent! Was the integral that you did

[tex]\int_0^\infty \frac{x}{2(x^2+2)^{\frac{7}{8}}} \ dx[/tex]

? Do you see why this is the correct integral, if not?
 

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