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sum_(k=1)^(oo)(-1)^(k)(3k-1)/(2k+1)

k=1(1)k3k12k+1 \sum_{k=1}^{\infty}(-1)^{k} \frac{3 k-1}{2 k+1}

Full solution

Q. k=1(1)k3k12k+1 \sum_{k=1}^{\infty}(-1)^{k} \frac{3 k-1}{2 k+1}
  1. Check Convergence: The given series is an alternating series of the form k=1(1)k(3k1)2k+1\sum_{k=1}^{\infty} \frac{(-1)^k(3k-1)}{2k+1}. To find the sum of an infinite series, we need to determine if the series converges and, if it does, use a method suitable for finding the sum of an alternating series.
  2. Calculate Limit: First, we need to check if the series converges. An alternating series converges if the absolute value of the terms tends to zero as kk tends to infinity and if the terms decrease in absolute value. Let's check the limit of the absolute value of the terms as kk approaches infinity.
  3. Evaluate Limit: We calculate the limit of the absolute value of the terms: limk3k12k+1\lim_{k\to\infty} \left|\frac{3k-1}{2k+1}\right|. As kk approaches infinity, the 1-1 and +1+1 become negligible, so the limit is essentially the limit of 3k2k\frac{3k}{2k}, which simplifies to 32\frac{3}{2}. Since the limit is not zero, the series does not meet the criteria for convergence of an alternating series.

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