Mathematics

Mathematics

Problem

A student asserts that the alternating series error bound may be used to estimate the remainder of ∑n=1∞(−1)nsin⁡(1/n)n\displaystyle \sum_{n=1}^{\infty} (-1)^{n}\dfrac{\sin(1/n)}{n} because the limit of the terms is zero. Which of the following comments most accurately evaluates the student’s assertion? a) The claim is valid because (−1)n(-1)^{n} ensures alternation and the limit of the terms is zero, so the next term always bounds the remainder. b) The claim is invalid; a decreasing sequence of term magnitudes is also required, and this has not been established. c) The claim fails because sin⁡(1/n)\sin(1/n) is always positive, meaning the series is not truly alternating. d) The claim is unnecessary; since ∑n=1∞sin⁡(1/n)n\displaystyle\sum_{n=1}^{\infty}\frac{\sin(1/n)}{n} converges absolutely, every remainder is automatically smaller than the next term.

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