Mathematics

Mathematics

Problem

A student wishes to analyze the convergence of the series ∑n=1∞(−1)n−1nn+1\displaystyle \sum_{n=1}^{\infty} (-1)^{n-1}\dfrac{\sqrt{n}}{n+1} by evaluating an improper integral. Which of the following approaches is valid? a) Integrate (−1)x−1xx+1(-1)^{x-1}\dfrac{\sqrt{x}}{x+1} from 11 to ∞\infty using the Integral Test. b) Integrate xx+1\dfrac{\sqrt{x}}{x+1} from 11 to ∞\infty and compare the result with the series of absolute values. c) Integrate xx+1\dfrac{\sqrt{x}}{x+1} from 11 to ∞\infty; since the integral diverges, conclude that the alternating series diverges as well. d) Replace xx+1\dfrac{\sqrt{x}}{x+1} by 1x\dfrac1{\sqrt{x}} and apply the pp–integral test, asserting convergence because p=12>1p=\tfrac12>1.

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