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AP Calculus AB: 12.3.3 Infinite Limits of Integration, Convergence, and Divergence

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This content explains how improper integrals are formally defined by replacing infinite limits or discontinuities with parameters and then taking the limit. It covers three types of improper integrals: integrals with an infinite upper limit, an infinite lower limit, or both limits extending to infinity. In each case, the improper integral is expressed as the limit of a proper integral. For discontinuous functions, the interval is split at the point of discontinuity, and limits are taken from each side to evaluate the integral.

Infinite Limits of Integration, Convergence, and Divergence

Improper integrals can be expressed as the limit of a proper integral as some parameter approaches either infinity or a discontinuity

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Key Terms

Term
Definition

Infinite Limits of Integration, Convergence, and Divergence

Improper integrals can be expressed as the limit of a proper integral as some parameter approaches either infinity or a discontinuity

note

  • Formalizing the idea of improper integrals involves
    replacing the infinite endpoint with a parameter whose limit approaches either infin...

Which of the following expressions is equivalent to the improper integral
∫∞af(x)dx?

limb→∞∫baf(x)dx

To evaluate the improper integral∫5−51x2dx,at which values of x should you break the integral?

x = 0

Evaluate ∫2−2 dx/x^4

The integral diverges.

Consider the red region under the curve y=f(x) where x→∞. Which of the following expressions correctly describes the limit of the area of the red region?

limb→∞∫baf(x)dx

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TermDefinition

Infinite Limits of Integration, Convergence, and Divergence

Improper integrals can be expressed as the limit of a proper integral as some parameter approaches either infinity or a discontinuity

note

  • Formalizing the idea of improper integrals involves
    replacing the infinite endpoint with a parameter whose limit approaches either infinity or the discontinuity.

  • There are three types of improper integrals over an infinite interval:

  • In the first integral to the left, the right endpoint is infinite. To formalize this integral is replaced with b and the integral is evaluated as .

  • In the second integral, the left endpoint approaches negative infinity. To formalize this integral – is replaced with a and the integral is evaluated as .

  • In the third integral, the range of integration is the entire x-axis. Split the integral into the sum of two integrals each of which has a limit of integration at some midpoint, t. The first integral can be evaluated as in example 2 above and the second can be evaluated as in example 1.

  • In this case the integral is improper because its domain has a discontinuity. Split the integral into the sum of two integrals each of which has a limit of integration at the discontinuity, x = c. The first integral is formalized by replacing c with E and evaluating the integral as . (Note that means E approaches c from the left or negative side of the x-axis.) The second integral is formalized by replacing c with D and evaluating the integral as . ( indicates that D approaches c from the right or positive side of the x-axis.)

Which of the following expressions is equivalent to the improper integral
∫∞af(x)dx?

limb→∞∫baf(x)dx

To evaluate the improper integral∫5−51x2dx,at which values of x should you break the integral?

x = 0

Evaluate ∫2−2 dx/x^4

The integral diverges.

Consider the red region under the curve y=f(x) where x→∞. Which of the following expressions correctly describes the limit of the area of the red region?

limb→∞∫baf(x)dx

Evaluate ∫∞−∞−11+x2dx

Evaluate∫1−11√1−x2dx.

π

Consider the red region under the curvey=f(x) where x→∞. Which of the following statements about the area isnot correct?

  • The area is equal to the improper integral
    ∫∞af(x)dx.

  • If the value of the improper integral is finite, then the integral converges.

  • If the value of the improper integral is infinite, then the integral diverges.

To evaluate the improper integral∫π0sec2xdx,at which values of x should you break the integral?

π / 2

Evaluate ∫∞0e−xdx.

1