Problem with improper integrals

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In summary, an improper integral is an integral with infinite limits of integration or an undefined integrand, which poses a problem because it fails to meet the conditions for Riemann integrability. To determine if it converges or diverges, specialized tests can be used. Traditional integration methods cannot be applied, and improper integrals have practical applications in fields such as mathematics, physics, and economics.
  • #1
cesaruelas
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Homework Statement



∫xe^(-2x)dx from x = 0 to ∞

Homework Equations



-xe^(-2x)/2 - e^(-2x)/4 + C

The Attempt at a Solution



lim b→∞ -x/2e^(2b) - 1/4e^(2b) = 0

wolfram alpha says its 1/4 and I do not know why (it does not show steps)

Can you help me?
 
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  • #2
You did not take the lower limit into account. Substitute x=0 and subtract.

ehild
 
  • #3
You're right lol I'm dumb... anyway substituting the lower limit I get 3/4 and not 1/4 :/
-(-1/2 - 1/4) = 3/4
 
  • #4
Thank you
 
  • #5
just saw my mistake... 1/2 should be zero, that way I do get 1/4... thank you again
 

FAQ: Problem with improper integrals

What is an improper integral?

An improper integral is an integral where one or both of the limits of integration are infinite, or the integrand is undefined at one or more points in the interval.

Why do improper integrals pose a problem?

Improper integrals pose a problem because they do not satisfy the basic conditions for Riemann integrability, which requires the function to be bounded and continuous on a closed interval. This can lead to unexpected and sometimes undefined results.

How do you determine if an improper integral converges or diverges?

To determine if an improper integral converges or diverges, you can use the limit comparison test, comparison test, or the integral test. These tests evaluate the behavior of the integral as the limits of integration approach infinity or negative infinity.

Can improper integrals be solved using traditional integration methods?

No, improper integrals cannot be solved using traditional integration methods like the power rule or substitution. Instead, they require specialized techniques such as partial fractions, trigonometric substitutions, or breaking the integral into smaller, convergent parts.

How are improper integrals used in real-world applications?

Improper integrals are used in many real-world applications, such as calculating areas under unbounded curves, finding the center of mass of an object, and determining the probability of continuous events in statistics. They also have uses in physics, engineering, and economics.

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