Estimating a limit from a table
Problem 1.653 · easy
Make a table of values of \( \displaystyle f(x) = \frac{x^{2} - 4 x}{x^{2} + 5 x} \) at \( \displaystyle x = 0 \pm 0.1,\ 0 \pm 0.01,\ 0 \pm 0.001,\ 0 \pm 0.0001 \) and use it to estimate \( \displaystyle \lim_{x \to 0} f(x) \).
- f(0) itself is undefined (0/0), so the table approaches 0 from both sides.
- The table: f(0 − 0.1) ≈ -8.36734694e-1; f(0 − 0.01) ≈ -8.03607214e-1; f(0 − 0.001) ≈ -8.00360072e-1; f(0 − 0.0001) ≈ -8.00036001e-1; f(0 + 0.0001) ≈ -7.99964001e-1; f(0 + 0.001) ≈ -7.99640072e-1; f(0 + 0.01) ≈ -7.96407186e-1; f(0 + 0.1) ≈ -7.64705882e-1.
- From both sides the values settle toward -0.800000.
- \[ \lim_{x \to 0}\left(\frac{x^{2} - 4 x}{x^{2} + 5 x}\right) = - \frac{4}{5} \]The exact (two-sided) limit agrees with the table.✓ Proved
Answer \( \lim_{x\to 0} f(x) = - \frac{4}{5} \approx -0.800000 \)
Lines: 1 proved, 3 not checked. 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 | Not checked | — | a sentence; read, not computed |
| 2 | Not checked | — | a sentence; read, not computed |
| 3 | Not checked | — | 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: fail (misleading) — Sentence 1 incorrectly states that the values approach 0; they actually approach –4/5. The statement about 0/0 is correct, but the conclusion about approaching 0 is false and would mislead a student.qwen3.6:27b-mlx: pass — The solution correctly identifies the indeterminate form, provides accurate numerical approximations consistent with the limit, and confirms the result with an exact calculation.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-18 — The solution correctly identifies the indeterminate form, provides accurate numerical approximations consistent with the limit, and confirms the result with an exact calculation.gpt-oss:20b: fail (misleading) 2026-10-18 — Sentence 1 incorrectly states that the values approach 0; they actually approach –4/5. The statement about 0/0 is correct, but the conclusion about approaching 0 is false and would mislead a student.qwen3.6:27b-mlx: pass 2026-10-16gpt-oss:20b: fail (misleading) 2026-10-16 — Sentence 1 incorrectly states that the table approaches 0 from both sides; it should say the values approach –0.8, not 0.qwen3.6:27b-mlx: pass 2026-10-14 — The solution correctly identifies the undefined nature of the function at x=0, provides accurate numerical approximations for the specified points, and correctly concludes the limit based on the trend. The final symbolic check confirms the result.gpt-oss:20b: fail (style) 2026-10-14 — [domain objection, downgraded to style] Sentence 1 incorrectly claims the table approaches 0 from both sides; it should state that the function is undefined at x=0 and the table approaches the limit -4/5.
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 2026-10-18 with SymPy 1.14.0.