Numerical Sequences and Series: The alternating series test

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In problems 1 – 4 determine if the series converges absolutely, converges conditionally, or diverges.

 

1. 1. 1 – 1/3 + 1/9 – 1/27 + 1/81 – …

Answer

1. The series converges absolutely.

Solution

1. To test absolute convergence, we test the series: 1 + |-1/3| + 1/9 + |-1/27| + 1/81 + … the geometric series with r = 1/3. This series converges, so the alternating series converges absolutely.

 

2. an= (-1)nn/(n2 + 2)

Answer

1. The series converges conditionally.

Solution

1. To test absolute convergence, test the series with terms |an| = n/(n2 + 2). The terms look like n/n2 = 1/n, which diverges. Because n/(n2 + 2) < 1/n, we cannot apply the Comparison Test. Instead, apply the Limit Comparison Test. limn → ∞ n/(n2 + 2)/(1/n)= limn → ∞ 1/(1 + 2/n2) = 1 Consequently, the series with terms |an| diverges. To test conditional convergence, we apply the Alternating Series Test. Consider the function f(x)= x/(x2 + 2) and compute f'(x)= (2 – x2)/(x2 + 2)2. Because f'(x) is negative for x > √2, the terms |an| are eventually decreasing. Also, limx → ∞ f(x)= 0, so limn → ∞,/sub> an= 0. Then by the Alternating Series Test, the series converges. Because it does not converge absolutely, the series converges conditionally.

 

3. an = (-1)n(n2 – 2n + 3)/(n2 + 2n + 5)

Answer

1. The series diverges.

Solution

1. The numerator and denominator have the same highest power of n, so apply the nth term test limn → ∞(n2 – 2n + 3)/(n2 + 2n + 5) = limn → ∞(1 – 2/n + 3/n2)/(1 + 2/n + 5/n2)= 1 Because the terms do not go to 0, the series diverges.

 

4. an= cos(n)/n2

Answer

1. The series converges absolutely.

Solution

1. Note |an| ≤ 1/n2, a p-series with p= 2, converging by the Integral Test. Because the series with term |an| converges, the original series converges absolutely.

 

5. For which p does the alternating p-series 1 – 1/2p + 1/3p – 1/4p + … converge absolutely, converge conditionally, or diverge?

Answer

1. Converges absolutely for p > 1, converges conditionally for 0 < p ≤ 1, and diverges for p ≤ 0.

Solution

1. To test absolute convergence, we test the series: 1 + |-1/2p| + |1/3p| + |-1/4p| + … the regular p-series. From the Integral Test, we know the p-series converges for p > 1 and diverges for p ≤ 1. Consequently, the alternating p-series converges absolutely for p > 1. For 0 < p ≤ 1, apply the Alternating Series Test. For f(x)= 1/xp, we find f'(x)= -p/xp+1 so f(x) is decreasing. Also, limn → ∞ 1/np= 0 so the alternating p-series converges. Because the series does not converge absolutely in this range of p-values, the series converges conditionally. For p ≤ 0, the series diverges by the nth term test.