Home›Calculus 2›Geometric series›Problem 7.149

Geometric series

Problem 7.149 · easy Video

Does \( \displaystyle \sum_{n=1}^{\infty} 3 \left(-1\right)^{n - 1} \) converge? If so, find its sum.

\[ 3 \]
The first term, a.Reviewed
\[ -1 \]
The common ratio, r.Reviewed
|r| = 1 ≥ 1, so the series diverges.
Reviewed

Answer \( \text{diverges} \)

Bars show the terms 3(-1)^(n - 1) for n = 1 to 24; dots show the running totals. The totals never settle: the series diverges.

Lines: 3 reviewed. The answer was also checked a second way, without looking at the solution. The reviewers disagree about how one step is explained; every verdict is in the receipt. How we verify

The full receipt

Line	Status	Checked by	Detail
1	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	both sides are the same expression: the line states a value, and the working that produced it is not on the line
2	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	both sides are the same expression: the line states a value, and the working that produced it is not on the line
3	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	a sentence; read, not computed
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	the terms do not shrink (60th term checked)

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly identifies the series as geometric with |r|=1 and applies the divergence criterion for geometric series. The conclusion is correct.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-09-28 — The solution correctly identifies the series as geometric with |r|=1 and applies the divergence criterion for geometric series. The conclusion is correct.
gpt-oss:20b: pass 2026-09-28
qwen3.6:27b-mlx: pass 2026-09-27 — The solution correctly identifies the series as geometric with |r|=1 and applies the divergence criterion for geometric series. The conclusion is correct.
gpt-oss:20b: pass 2026-09-27
qwen3.6:27b-mlx: inconclusive 2026-09-27 — reviewer response could not be parsed: { "verdict": "fail", "severity": "error", "notes": "The solution incorrectly identifies the first term as 3 (it is 3) and the common ratio as -1, but the logic for divergence is flawed because the geo
gpt-oss:20b: pass 2026-09-27

Proved: SymPy reduced the difference between the two sides to zero. Checked independently: a separate method, named above, confirmed it. Checked numerically: the two sides agree at every sampled point, which is evidence, not proof. Reviewed: a model or a person read it; that is all a sentence can have. Solution by generator:structured/geometric_series, checked 2026-10-13 with SymPy 1.14.0.