Integration Problem: Solving $\int_{0}^{\infty}x^3.e^{-x^2}\mathrm{d}x$

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In summary, the conversation is about attempting to solve the integral \int_{0}^{\infty}x^3.e^{-x^2} \mathrm{d}x using various methods, including substitution, integration by parts, and partial fraction decomposition. The conversation ends with one person realizing their mistake and thanking the other for their help. The conversation also touches on the dislike of calculus and the importance of practice in becoming a mathematician.
  • #1
qspeechc
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Homework Statement



[tex] \int_{0}^{\infty}x^3.e^{-x^2} \mathrm{d}x [/tex]

The Attempt at a Solution



I have tried substitution u=x^2, u=x^3; integration by parts; squeeze theorem; partial fraction decomp; taylor series expansion- but nothing seems to work. I know the limit of [tex]x^3.e^{-x^2}[/tex] as x tends to infinity is zero, but that doesn't help.
Any help please?
 
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  • #2
qspeechc said:
I have tried substitution u=x^2
That should work.
 
  • #3
Well, ok.

Let u = x^2 , then: du = 2x.dx. And then what? We have an x^3 in the integration, so I don't see how it works.
 
  • #4
But x3 = x.x2, isn't it?
 
  • #5
Yes, but then we would have:

[tex]x^2.e^{-u}.du[/tex] or am I missing something?
 
  • #6
qspeechc said:
[tex]x^2.e^{-u}.du[/tex] or am I missing something?
You just did the sub u=x^2 a couple of steps ago.
 
  • #7
AHA! Oh thank-you neutrino! I can't believe I never saw that! Gee, I feel like an idiot! Thank-you again!
 
  • #8
qspeechc said:
AHA! Oh thank-you neutrino! I can't believe I never saw that! Gee, I feel like an idiot! Thank-you again!

You're welcome. Make a hobby out of solving integrals (if you're into those kind of things), and you'll start recognising the methods with just a look at the integral. (For some of them, at least. :biggrin:)
 
  • #9
Actually I quite dislike calculus. I find it dry and boring. Or atleast that's the way my first year course presents it. But I guess you are right, I need to do more calculus problems if I want to be a mathematician :-p, which I do.
 

FAQ: Integration Problem: Solving $\int_{0}^{\infty}x^3.e^{-x^2}\mathrm{d}x$

How do you solve the integration problem, $\int_{0}^{\infty}x^3.e^{-x^2}\mathrm{d}x$?

To solve this integration problem, we can use the substitution method. Let u = x^2, then du = 2x dx. We can rewrite the integral as $\frac{1}{2} \int_{0}^{\infty}u.e^{-u}\mathrm{d}u$. We can then use integration by parts to solve for the antiderivative, which is $\frac{1}{2}(-e^{-u}(u+1))$. Substituting back in for u, we get the final answer of $\frac{1}{2}(-e^{-x^2}(x^2+1))$.

What is the significance of the upper limit being infinity in this integration problem?

The upper limit of infinity in this integration problem signifies that we are integrating over an unbounded interval. In other words, we are finding the area under the curve of the function from 0 to infinity.

Can this integration problem be solved using other methods besides substitution?

Yes, there are other methods that can be used to solve this integration problem. Some examples include using partial fractions or trigonometric substitutions.

Is there a way to check if the solution to this integration problem is correct?

Yes, we can check the solution by taking the derivative of the antiderivative we found and seeing if it matches the original integrand. In this case, the derivative of $\frac{1}{2}(-e^{-x^2}(x^2+1))$ is indeed $x^3.e^{-x^2}$, so our solution is correct.

Can this integration problem be solved by hand or is a computer needed?

This integration problem can be solved by hand, as shown in the previous steps. However, a computer can also be used to solve it numerically, which may be necessary for more complex integration problems.

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