Improper Integrals and Their Convergence · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Improper integrals and finite limits

Mathematics · Calculus & Analysis · ages 19-20
Name ______________________   Date ____________
  1. Let f(x)=1/(x-2)^2. Which setup for ∫0^6 f(x) dx uses the required independent one-sided limits?

    • lim(t -> 6 from below) ∫0^t f(x) dx, keeping the interior singularity inside each interval
    • lim(u -> 2 from below) ∫0^u f(x) dx + lim(v -> 2 from above) ∫v^6 f(x) dx
    • lim(a -> 0 from above) ∫a^6 f(x) dx, approaching the regular left endpoint instead
    • lim(h -> 0 from above) [∫0^(2-h) f(x) dx + ∫(2+h)^6 f(x) dx], requiring the cutoffs to move together
  2. Let f be continuous on [1, infinity). Which limit must be tested for its improper integral on this interval?

    • Substitute infinity into f(x) and call that value the integral
    • Take lim(t -> infinity) of the integral from 1 to t
    • Take lim(t -> 1 from above) of the integral from 1 to t
  3. For which p does the integral from 1 to infinity of 1/x^p dx converge?

    • p < 1
    • p = 1
    • p > 1
  4. Evaluate the integral from 1 to infinity of 1/x^2 dx.

    Answer: ______________

  5. Evaluate the integral from 0 to 1 of 1/sqrt(x) dx.

    Answer: ______________

  6. Let f(x)=x/(1+x^2). For every t>0, ∫-t^t f(x) dx=0, while ∫0^b f(x) dx=ln(1+b^2)/2. What follows about ∫-infinity^infinity f(x) dx?

    • It diverges because the right-hand limit is not finite.
    • It converges to 0 because every symmetric cutoff gives 0.
    • It converges to 1 because each denominator contains 1.
    • It has value 0 because opposite infinite tails always cancel.
  7. For real p>1, ∫1^infinity 1/x^p dx has value 3. What is p?

    • p = 1/3
    • p = 3/2
    • p = 4/3
    • p = 5/3
  8. Evaluate the integral from 1 to infinity of 1/x^3 dx. Enter a decimal.

    Answer: ______________

  9. A friend says ∫1^infinity 1/x dx is finite because 1/x tends to 0. Which check settles this claim?

    • Since 1/x never equals 0, every finite interval has area 0
    • The integral to t is ln(t), which grows without bound
    • Since 1/x tends to 0, its integral must have the same limit
  10. Let f be continuous on the real line. If ∫-infinity^0 f(x) dx and ∫0^infinity f(x) dx each converge, then ∫-infinity^infinity f(x) dx converges.

    Circle one:   True   False

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Answer key

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Improper integrals and finite limits W1-mt_gvtZPFRA6Z-s1

  1. lim(u -> 2 from below) ∫0^u f(x) dx + lim(v -> 2 from above) ∫v^6 f(x) dx · The singularity is at 2. Use independent cutoffs approaching it from below and above. Test each limit separately before adding values.
  2. Take lim(t -> infinity) of the integral from 1 to t · Integrate to a finite upper cutoff t, then take its limit as t grows without bound.
  3. p > 1 · The p rule says 1/x^p on [1, infinity) converges exactly when p is greater than 1.
  4. 1 · Using the antiderivative -1/x, the integral from 1 to t is 1-1/t. Its limit is 1.
  5. 2 · Using the antiderivative 2 sqrt(x), the integral from a small a to 1 approaches 2.
  6. It diverges because the right-hand limit is not finite. · The right-hand integral grows without bound. Whole-line convergence requires both independent tail limits to be finite; symmetric cancellation does not establish this.
  7. p = 4/3 · For p>1, the integral is 1/(p-1). Solving 1/(p-1)=3 gives p-1=1/3, so p=4/3.
  8. 0.5 · Here p = 3 is greater than 1, and the limit is 1/2.
  9. The integral to t is ln(t), which grows without bound · The finite integral is ln(t). It grows without bound, so the improper integral diverges even though 1/x tends to 0.
  10. True · Each independent tail limit is finite. Their sum defines the convergent whole-line improper integral.
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