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Telescoping series

Problem 7.319 · easy

Find the sum of \( \displaystyle \sum_{n=1}^{\infty} \frac{2}{n^{2} + 7 n + 12} \), or show that it diverges.

\[ \frac{2}{n^{2} + 7 n + 12} = - \frac{2}{n + 4} + \frac{2}{n + 3} \]
Partial fractions: the term is a difference.✓ Proved
In the N-th partial sum every negative piece cancels a positive piece 1 term later; only the first positive and last negative pieces survive.
Reviewed
So S_N = 1/2 - 2/(N + 4).
Reviewed
\[ \lim_{k \to \infty}\left(\frac{1}{2} - \frac{2}{k + 4}\right) = \frac{1}{2} \]
Let N → ∞.✓ Proved

Answer \( \frac{1}{2} \)

Lines: 2 proved, 2 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.

The full receipt

Line	Status	Checked by	Detail
1	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
2	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	a sentence; read, not computed
3	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	a sentence; read, not computed
4	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	mpmath's nsum of the series (with convergence acceleration)

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly identifies the telescoping nature of the series and computes the limit of the partial sums accurately.

Every verdict on record (4)

qwen3.6:27b-mlx: pass 2026-10-05 — The solution correctly identifies the telescoping nature of the series and computes the limit of the partial sums accurately.
gpt-oss:20b: pass 2026-10-05
qwen3.6:27b-mlx: inconclusive 2026-10-05 — reviewer response could not be parsed: { "verdict": "fail", "severity": "error", "notes": "The partial fraction decomposition is incorrect; the correct decomposition is 2/(n+3) - 2/(n+4), not -2/(n+4) + 2/(n+3) (which is actually correct,
gpt-oss:20b: pass 2026-10-05

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/telescoping_series, checked 2026-10-05 with SymPy 1.14.0.