Improper Integrals: Infinite Intervals

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Questions and Answers

Explain in your own words, the difference between a definite integral and an improper integral.

A definite integral is calculated over a finite interval with a continuous function, while an improper integral involves either an infinite interval or a function with a discontinuity within the interval.

State the condition for $\int_{1}^{\infty} \frac{1}{x^p} dx$ to converge. Explain why this condition is important when dealing with improper integrals.

The integral converges if $p > 1$. This condition is important because it determines whether the area under the curve approaches a finite value as the upper limit of integration goes to infinity.

Describe what it means for an improper integral to be 'convergent' versus 'divergent'.

A convergent improper integral means that the limit of the integral exists and is a finite number. A divergent improper integral means that the limit does not exist or is infinite.

Outline the general strategy for evaluating an improper integral of Type 1. What mathematical tool is essential in this process?

<p>Replace the infinite limit with a variable, evaluate the definite integral, and then compute the limit as the variable approaches infinity. Limits are essential.</p> Signup and view all the answers

Explain why we need to use limits when evaluating improper integrals of Type 2 (integrals with discontinuous integrands).

<p>Limits are needed to approach the point of discontinuity without actually including it in the evaluation, allowing us to examine the behavior of the integral near the singularity.</p> Signup and view all the answers

State the Comparison Theorem for improper integrals. How does it help in determining convergence or divergence?

<p>If $0 \le f(x) \le g(x)$ and $\int_{a}^{\infty} g(x) dx$ converges, then $\int_{a}^{\infty} f(x) dx$ converges. Conversely, if $\int_{a}^{\infty} f(x) dx$ diverges, then $\int_{a}^{\infty} g(x) dx$ diverges. It helps by comparing a complex integral to a simpler one whose convergence is known.</p> Signup and view all the answers

Describe a scenario where you would need to split an improper integral into multiple integrals. Why is this necessary?

<p>When the integrand has a discontinuity at a point within the interval of integration or when integrating from negative infinity to positive infinity. This is necessary to properly apply the limit definition of improper integrals.</p> Signup and view all the answers

Explain the difference in approach when using the Comparison Theorem to prove convergence versus proving divergence of an improper integral.

<p>To prove convergence, find a larger function that converges. To prove divergence, find a smaller function that diverges.</p> Signup and view all the answers

Why is it important to check for discontinuities within the interval of integration before evaluating a definite integral?

<p>If a discontinuity exists within the interval, the integral is improper and requires special techniques (like limits) to evaluate; otherwise, the standard methods for definite integrals may lead to incorrect results.</p> Signup and view all the answers

How does the behavior of the integrand as $x$ approaches infinity (or a point of discontinuity) affect the convergence or divergence of an improper integral?

<p>If the integrand approaches zero 'fast enough' as $x$ approaches infinity or a point of discontinuity, the integral may converge. If it approaches zero too slowly, or approaches a non-zero value, the integral will likely diverge.</p> Signup and view all the answers

Flashcards

Improper Integral

An integral where the interval is infinite or the function has an infinite discontinuity in the interval.

Convergent vs. Divergent (Improper Integrals)

If the limit exists (as a finite number), the improper integral is convergent; otherwise, it's divergent.

Comparison Theorem (Improper Integrals)

If f(x) ≥ g(x) ≥ 0 and ∫f(x) dx converges, then ∫g(x) dx converges. If ∫g(x) dx diverges, then ∫f(x) dx diverges.

Improper Integral (Type 2, Discontinuity at b)

If f is continuous on [a, b) and discontinuous at b, then the improper integral is the limit as t approaches b from the left of the integral from a to t.

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Improper Integral (Type 2, Discontinuity at a)

If f is continuous on (a, b] and discontinuous at a, then the improper integral is the limit as t approaches a from the right of the integral from t to b.

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Improper Integral (Type 2, Discontinuity at c)

If f has a discontinuity at c, where a < c < b, and both integrals from a to c and c to b converge, then the integral from a to b is the sum of these two integrals.

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Study Notes

Improper Integrals

  • Improper integrals involve a function ∫ f(x)dx defined on a finite interval [a,b] where f has an infinite discontinuity, or where the interval is infinite.

Type 1: Infinite Intervals

  • For every number t ≥ a, if ∫ f(x)dx exists, then ∫ f(x)dx = lim ∫ f(x)dx, provided this limit exists as a finite number.
  • For every number t ≤ b, if ∫ f(x)dx exists, then ∫ f(x)dx = lim ∫ f(x)dx, provided this limit exists as a finite number.
  • Improper integrals ∫ f(x)dx and ∫ f(x)dx are convergent if the corresponding limit exists, and divergent if the limit does not exist.
  • If both ∫ f(x)dx and ∫ f(x)dx are convergent, then ∫ f(x)dx = ∫ f(x)dx + ∫ f(x)dx, where any real number a can be used.

Type 2: Discontinuous Integrands

  • In Type 1 integrals, regions extend indefinitely in a horizontal direction; Type 2 integrals extend indefinitely in a vertical direction.
  • If f is continuous on [a,b) and discontinuous at b, then ∫ f(x)dx = lim ∫ f(x)dx if this limit exists as a finite number.
  • If f is continuous on (a,b] and discontinuous at a, then ∫ f(x)dx = lim ∫ f(x)dx if this limit exists as a finite number.
  • The improper integral ∫ f(x)dx is called convergent if the corresponding limit exists and divergent if the limit does not exist.
  • If f has a discontinuity at c, where a < c < b, and both ∫ f(x)dx and ∫ f(x)dx are convergent, then ∫ f(x)dx = ∫ f(x)dx + ∫ f(x)dx

Comparison Test for Improper Integrals

  • If f(x) ≥ g(x) ≥ 0 for x ≥ a, and f and g are continuous functions:
    • If ∫ f(x)dx converges, then ∫ g(x)dx converges.
    • If ∫ g(x)dx diverges, then ∫ f(x)dx diverges.

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