Limit of \( \displaystyle \frac{4 x - 1}{\left(2 x - 1\right)^{2} + 3} \) as \( x \to \infty \)
Problem 1.418 · medium
- \[ \lim_{x \to \infty}\left(\frac{4 x - 1}{\left(2 x - 1\right)^{2} + 3}\right) \]limitStart with the original limit.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{4 x - 1}{4 x^{2} - 4 x + 4}\right) \]algebra simplifyExpand the denominator. Combine the constant terms.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{\frac{4}{x} - \frac{1}{x^{2}}}{4 - \frac{4}{x} + \frac{4}{x^{2}}}\right) \]rewriteDivide numerator and denominator by x**2.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{4}{x} - \frac{1}{x^{2}}\right) \left(\lim_{x \to \infty}\left(4 - \frac{4}{x} + \frac{4}{x^{2}}\right)\right)^{-1} \]limit-lawApply the quotient rule for limits.✓ Proved
- \[ = 0 \]limit simplifyEvaluate the limits of the numerator and denominator. Final result.✓ Proved
✓ Nihil obstat Every line of this solution was proved by the computer algebra system SymPy. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | line 1 is the problem as stated |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where (2*x - 1)**2 + 3 = 0 undefined where 4*x**2 - 4*x + 4 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 - 4*x + 4 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 4*x**2 - 4*x + 4 = 0 undefined where x = 0 undefined where 4 - 4/x + 4/x**2 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where 4 - 4/x + 4/x**2 = 0 undefined where Limit(4 - 4/x + 4/x**2, x, oo, dir='-') = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where Limit(4 - 4/x + 4/x**2, x, oo, dir='-') = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| answer | ✓ Proved | sympy 1.14.0 | final line against the stated answer: simplify(a - b) reduced to 0 |
| answer, a second way | ✓ Proved | sympy 1.14.0 | SymPy took the limit from both sides and got the stated value |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass
Senior review claude-sonnet-5-5, 2026-10-08: pass — Every step is a correct equivalence with a legitimate rule name. The limit is 0 (numerator tends to 0, denominator to 4), matching the stated answer.
qwen3.6:27b-mlx: dismiss — The objection is false: dividing 4*x - 1 by x**2 gives 4/x - 1/x**2, which is exactly line 4's numerator, and the denominator becomes 4 - 4/x + 4/x**2, so both were divided by x**2 and SymPy proved the line.
Every verdict on record (5)
gpt-oss:20b: pass 2026-10-08qwen3.6:27b-mlx: pass 2026-10-08claude-sonnet-5-5: pass 2026-10-08 — Every step is a correct equivalence with a legitimate rule name. The limit is 0 (numerator tends to 0, denominator to 4), matching the stated answer.qwen3.6:27b-mlx: inconclusive 2026-10-08 — reviewer response could not be parsed: { "verdict": "fail", "severity": "error", "notes": "Step 4 claims to divide numerator and denominator by x**2, but the numerator (4*x - 1) was only divided by x, resulting in (4/x - 1/x**2) instead ofgpt-oss:20b: pass 2026-10-08
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
gemma4:26b, checked 2026-10-08 with SymPy 1.14.0.