Estimating a limit from a table
Problem 1.720 · easy
Make a table of values of \( \displaystyle f(x) = \frac{x^{2} - 1}{x^{2} + 4 x + 3} \) at \( \displaystyle x = -1 \pm 0.1,\ -1 \pm 0.01,\ -1 \pm 0.001,\ -1 \pm 0.0001 \) and use it to estimate \( \displaystyle \lim_{x \to -1} f(x) \).
- f(-1) itself is undefined (0/0), so the table approaches -1 from both sides.Reviewed
- The table: f(-1 − 0.1) ≈ -1.10526316; f(-1 − 0.01) ≈ -1.01005025; f(-1 − 0.001) ≈ -1.0010005; f(-1 − 0.0001) ≈ -1.00010001; f(-1 + 0.0001) ≈ -9.99900005e-1; f(-1 + 0.001) ≈ -9.990005e-1; f(-1 + 0.01) ≈ -9.90049751e-1; f(-1 + 0.1) ≈ -9.04761905e-1.Reviewed
- From both sides the values settle toward -1.000000.Reviewed
- \[ \lim_{x \to -1}\left(\frac{x^{2} - 1}{x^{2} + 4 x + 3}\right) = -1 \]The exact (two-sided) limit agrees with the table.✓ Proved
Answer \( \lim_{x\to -1} f(x) = -1 \approx -1.000000 \)
Lines: 1 proved, 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 | a sentence; read, not computed |
| 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 | f evaluated at a ± 10⁻⁹ in 30-digit arithmetic |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly identifies the indeterminate form, provides accurate numerical values approaching the limit from both sides, and correctly concludes the limit is -1.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-19 — The solution correctly identifies the indeterminate form, provides accurate numerical values approaching the limit from both sides, and correctly concludes the limit is -1.gpt-oss:20b: pass 2026-10-19qwen3.6:27b-mlx: pass 2026-10-18 — The solution correctly identifies the indeterminate form, provides accurate numerical values approaching the limit from both sides, and confirms the result with symbolic computation.gpt-oss:20b: pass 2026-10-18qwen3.6:27b-mlx: inconclusive 2026-10-18 — reviewer response could not be parsed: { "verdict": "fail", "severity": "error", "notes": "The table values for x > -1 are incorrect (e.g., f(-0.9) is approximately -0.90476, not -0.90476... wait, let me re-calculate. f(-0.9) = ((-0.9)^2 -gpt-oss:20b: pass 2026-10-18
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/limit_table, checked 2026-10-19 with SymPy 1.14.0.