Infinite series and convergence tests

Practice sheet

Fresh questions from the whole chapter, to do on paper.

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  1. Does ∑n=1∞6nn+3\displaystyle\sum_{n=1}^{\infty} \dfrac{ 6n}{n + 3} converge?
    1. No: the terms tend to 6 ≠ 0, so it diverges (nth-term test)
    2. Yes, because each term is less than 6
    3. The nth-term test cannot decide
  2. ∑n=1∞4n\displaystyle\sum_{n=1}^{\infty} \dfrac{ 4 }{n}: its terms tend to 0. What does that tell you?
    1. Nothing by itself: this series in fact diverges (harmonic series)
    2. The series converges
    3. The sum is 0
  3. Find ∑n=1∞(9n−9n+1)\displaystyle\sum_{n=1}^{\infty} \left(\dfrac{ 9 }{n} - \dfrac{ 9 }{n + 1}\right).
  4. Use the integral test on ∑n=1∞7ne−n2\displaystyle\sum_{n=1}^{\infty} 7n e^{-n^2}.
    1. It converges: the integral of 7xe^(−x²) from 1 to ∞ is finite
    2. It diverges, because n grows
    3. The integral test does not apply
  5. The integral test bounds ∑k=2∞1k6\displaystyle\sum_{k=2}^{\infty} \dfrac{1}{k^{ 6 } } by ∫1∞dxx6\displaystyle\int_1^{\infty} \dfrac{dx}{x^{ 6 } }. Find that integral.
  6. Does ∑n=1∞1n2+7\displaystyle\sum_{n=1}^{\infty} \dfrac{1}{n^2 + 7} converge?
    1. Yes: it is smaller than the convergent ∑ 1/n²
    2. No: it is smaller than the divergent ∑ 1/n
    3. No test applies
  7. Does ∑n=1∞1n+8\displaystyle\sum_{n=1}^{\infty} \dfrac{1}{\sqrt{n} + 8} converge?
    1. No: it behaves like ∑ 1/√n, which diverges (limit comparison)
    2. Yes: it is smaller than ∑ 1/√n
    3. Yes: its terms tend to 0
  8. Does ∑n=1∞2n+1n3+8\displaystyle\sum_{n=1}^{\infty} \dfrac{2n + 1}{n^3 + 8} converge?
    1. Yes: it behaves like ∑ 2/n², a convergent p-series
    2. No: the top grows
    3. No: it behaves like ∑ 1/n
  9. Does ∑n=1∞(−1)n+17n\displaystyle\sum_{n=1}^{\infty} \dfrac{(-1)^{n+1} 7 }{n} converge?
    1. Yes, conditionally: the terms alternate, shrink and tend to 0
    2. No: ∑ 7/n diverges
    3. Yes, absolutely
  10. S=∑n=1∞(−1)n+1n2S = \displaystyle\sum_{n=1}^{\infty} \dfrac{(-1)^{n+1} }{n^2} is approximated by its first 8 terms. Give the alternating-series bound on the error.
  11. Find the partial sum S3S_3 of ∑n=1∞(−1)n+11n\displaystyle\sum_{n=1}^{\infty} \dfrac{(-1)^{n+1} 1 }{n}.
  12. For an=n8na_n = \dfrac{n}{ 8^n}, find L=lim⁡n→∞∣an+1an∣L = \displaystyle\lim_{n \to \infty} \left|\dfrac{a_{n+1} }{a_n}\right|.