Alternating Series and Absolute Convergence · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

When signs keep switching

Mathematics · Calculus & Analysis · ages 19-20
Name ______________________   Date ____________
  1. Why does the alternating harmonic series, with terms (minus 1) to the n over n, converge?

    • Its magnitudes decrease to zero
    • Its terms are all positive
    • Its partial sums are bounded by 1
    • The ratio test gives a limit of 0
  2. Why does the alternating harmonic series converge?

    • Its magnitudes decrease to zero
    • Its terms are all positive
    • The ratio test gives a limit of 0
  3. Stopping an alternating series after 10 terms leaves error at most the 11th magnitude.

    Circle one:   True   False

  4. Terms minus 1 to the n over n squared converge absolutely. Type p of the absolute p-series.

    Answer: ______________

  5. An alternating series has decreasing magnitudes. A partial sum through 4 terms omits from term 5 on. Type the error bound as a multiple of the 5th magnitude.

    Answer: ______________

  6. Approximating the alternating harmonic series with its first 10 terms leaves an error of at most what?

    • 1/11
    • 1/10
    • 1/100
    • 0
  7. Which series converges, but its absolute version diverges?

    • sum (minus 1) to the n over n
    • sum 1 over n squared
    • sum (1/2) to the n
    • sum n over 2 to the n
  8. Which series converges, but its absolute version diverges?

    • sum 1 over n squared
    • sum minus 1 to the n over n
    • sum 1 over 2 to the n
  9. Ten terms of the alternating harmonic series leave an error of at most what?

    • 1 over 11
    • 1 over 10
    • 1 over 100
  10. A student says the alternating harmonic series converges, so the plain harmonic series must converge too. What is the error?

    • The alternating test also proves the harmonic series converges
    • Removing the signs can break convergence; that gap is conditional convergence
    • The harmonic terms do not tend to zero
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Answer key

For grown-ups. Fold this page away before handing over the rest.

When signs keep switching W1-mt_sk6F4n1USH-s1

  1. Its magnitudes decrease to zero · Magnitudes 1 over n decrease monotonically to 0, which is exactly what the alternating series test needs.
  2. Its magnitudes decrease to zero · Magnitudes 1 over n decrease to 0, which is exactly what the test needs.
  3. True · The truncation error never exceeds the next magnitude.
  4. 2 · Absolute values give 1 over n squared, with p equals 2.
  5. 1 · The bound equals 1 times the first omitted magnitude.
  6. 1/11 · The error bound is the first omitted magnitude, and the 11th term has magnitude 1 over 11.
  7. sum (minus 1) to the n over n · The alternating harmonic series converges by the alternating test, while the plain harmonic series diverges.
  8. sum minus 1 to the n over n · The alternating harmonic converges, while the plain harmonic diverges.
  9. 1 over 11 · The error bound is the first omitted magnitude, the 11th term.
  10. Removing the signs can break convergence; that gap is conditional convergence · Convergence need not survive stripping the signs away.
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