Limit of \( \displaystyle \frac{4 x^{2} - 1}{2 x^{2} + x} \) as \( x \to \infty \)
Problem 1.22 · medium
Evaluate \( \displaystyle \lim_{x \to \infty} \frac{4 x^{2} - 1}{2 x^{2} + x} \).
- \[ \lim_{x \to \infty}\left(\frac{4 x^{2} - 1}{2 x^{2} + x}\right) \]limit algebraStart with the original limit expression. Divide both numerator and denominator by x**2.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{4 - \frac{1}{x^{2}}}{2 + \frac{1}{x}}\right) \]algebraSimplify the fractions inside the limit.✓ Proved
- \[ = \lim_{x \to \infty}\left(4 - \frac{1}{x^{2}}\right) \left(\lim_{x \to \infty}\left(2 + \frac{1}{x}\right)\right)^{-1} \]limit-lawApply the quotient rule for limits.✓ Proved
- \[ = 2 \]limit simplifyEvaluate the limits of the numerator and denominator separately. Final simplification.✓ Proved
Answer \( 2 \)
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. 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 | line 1 is the problem as stated |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x**2 + x = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x**2 + x = 0 undefined where 2 + 1/x = 0 undefined where x = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2 + 1/x = 0 undefined where x = 0 undefined where Limit(2 + 1/x, x, oo, dir='-') = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Limit(2 + 1/x, x, oo, dir='-') = 0 undefined where x = 0 |
| 6 | ✓ 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
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-09-29gpt-oss:20b: pass 2026-09-29qwen3.6:27b-mlx: fail (error) 2026-09-29 — Step 5 claims to evaluate the limits of the numerator and denominator separately, but it skips the intermediate step of evaluating the individual limits (e.g., Limit(4 - 1/x**2, x, oo) -> 4). It jumps directly from the limit expressions to the constants, violating the 'one thing per step' rule by combining limit evaluation and substitution.gpt-oss:20b: pass 2026-09-29
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 2026-09-29 with SymPy 1.14.0.