Question:hard

The series \(\dfrac{1}{2} - \dfrac{2}{3}\cdot\dfrac{1}{2^3} + \dfrac{3}{4}\cdot\dfrac{1}{3^3} - \dfrac{4}{5}\cdot\dfrac{1}{4^3} + \cdots\) is:

Show Hint

Write the general term as \((-1)^{n+1}\dfrac{n}{(n+1)n^3}\) and compare \(|a_n|\) with \(1/n^3\).
Updated On: Jul 4, 2026
  • Conditionally convergent
  • Absolutely convergent
  • Divergent
  • None of the above
Show Solution

The Correct Option is B

Solution and Explanation

Step 1: Write the general term of the series of absolute values as $b_n = \dfrac{1}{n^2(n+1)}$ and decompose it using partial fractions: $b_n = \dfrac{1}{n^2} + \left(\dfrac{1}{n+1} - \dfrac{1}{n}\right)$.
Step 2: Sum over $n=1$ to $N$: $\sum_{n=1}^N b_n = \sum_{n=1}^N \dfrac{1}{n^2} + \sum_{n=1}^N \left(\dfrac{1}{n+1}-\dfrac{1}{n}\right)$. The second sum telescopes to $\dfrac{1}{N+1} - 1$.
Step 3: Taking $N \to \infty$, $\sum_{n=1}^\infty \dfrac{1}{n^2} = \dfrac{\pi^2}{6}$ (a known convergent value) and the telescoped part tends to $-1$. So $\sum_{n=1}^\infty b_n = \dfrac{\pi^2}{6} - 1$, a finite number.
Step 4: Since the sum of absolute values is finite, the original alternating series converges absolutely, not just conditionally.
\[\boxed{\text{The series is absolutely convergent}}\]
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