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Limit of \( \displaystyle \frac{4 \left(x - 1\right)^{2} - 1}{x + 2 \left(x - 1\right)^{2} - 1} \) as \( x \to \infty \)

Problem 1.306 · hard

Evaluate \( \displaystyle \lim_{x \to \infty} \frac{4 \left(x - 1\right)^{2} - 1}{x + 2 \left(x - 1\right)^{2} - 1} \).
  1. \[ \lim_{x \to \infty}\left(\frac{4 \left(x - 1\right)^{2} - 1}{x + 2 \left(x - 1\right)^{2} - 1}\right) \]
    limitEvaluate the limit of the given function at infinity.✓ Proved
  2. \[ = \lim_{x \to \infty}\left(\frac{4 x^{2} - 8 x + 3}{2 x^{2} - 3 x + 1}\right) \]
    algebra simplifyExpand the squared terms. Simplify the numerator and denominator.✓ Proved
  3. \[ = \lim_{x \to \infty}\left(\frac{\frac{d}{d x} \left(4 x^{2} - 8 x + 3\right)}{\frac{d}{d x} \left(2 x^{2} - 3 x + 1\right)}\right) \]
    lhopitalApply L'Hopital's rule because the limit is of the form infinity/infinity.✓ Proved
  4. \[ = \lim_{x \to \infty}\left(\frac{8 x - 8}{4 x - 3}\right) \]
    simplifyCompute the derivatives.✓ Proved
  5. \[ = \lim_{x \to \infty}\left(\frac{\frac{8}{x} - \frac{8}{x^{2}}}{\frac{4}{x} - \frac{3}{x^{2}}}\right) \]
    algebraDivide both numerator and denominator by x.✓ Proved
  6. \[ = \lim_{x \to \infty}\left(\frac{8}{x}\right) \left(\lim_{x \to \infty}\left(\frac{4}{x}\right)\right)^{-1} \]
    limit-lawApply the quotient rule for limits, noting that the individual limits are 0.Not checked
  7. \[ = \lim_{x \to \infty} 8 \left(\lim_{x \to \infty} 4\right)^{-1} \]
    limit-lawMultiply numerator and denominator by x to resolve the 0/0 form.Not checked
  8. \[ = 2 \]
    limitEvaluate the resulting limit.✓ Proved
Answer \( 2 \)

Lines: 8 proved, 2 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.

The full receipt
LineStatusChecked byDetail
1✓ Provedsympy 1.14.0line 1 is the problem as stated
2✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where x + 2*(x - 1)**2 - 1 = 0
undefined where 2*x**2 - 3*x + 1 = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x**2 - 3*x + 1 = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x**2 - 3*x + 1 = 0
undefined where Derivative(2*x**2 - 3*x + 1, x) = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Derivative(2*x**2 - 3*x + 1, x) = 0
undefined where 4*x - 3 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 4*x - 3 = 0
undefined where 4/x - 3/x**2 = 0
undefined where x = 0
7Not checked—simplify left nan; no point in the sample was defined on both lines
undefined where 4/x - 3/x**2 = 0
undefined where x = 0
undefined where Limit(4/x, x, oo, dir='-') = 0
8Not checked—simplify left nan; no point in the sample was defined on both lines
undefined where x = 0
undefined where Limit(4/x, x, oo, dir='-') = 0
undefined where Limit(4, x, oo, dir='-') = 0
9✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Limit(4, x, oo, dir='-') = 0
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
answer, a second way✓ Provedsympy 1.14.0SymPy took the limit from both sides and got the stated value

Reviewers

  • gpt-oss:20b: pass
  • qwen3.6:27b-mlx: fail (error) — Step 8 is mathematically invalid. The previous step (Step 7) incorrectly applied the quotient rule for limits to a form where the denominator's limit is 0 (Limit(4/x, x, oo) = 0), which is not allowed. Furthermore, Step 8 attempts to 'multiply by x' to resolve a non-existent 0/0 form in the limit expression, resulting in an incorrect transition from 8/x to 8.
Every verdict on record (4)
  • gpt-oss:20b: pass 2026-10-06
  • qwen3.6:27b-mlx: fail (error) 2026-10-06 — Step 8 is mathematically invalid. The previous step (Step 7) incorrectly applied the quotient rule for limits to a form where the denominator's limit is 0 (Limit(4/x, x, oo) = 0), which is not allowed. Furthermore, Step 8 attempts to 'multiply by x' to resolve a non-existent 0/0 form in the limit expression, resulting in an incorrect transition from 8/x to 8.
  • qwen3.6:27b-mlx: fail (error) 2026-10-06 — Step 7 incorrectly applies the quotient rule for limits. The limit of a quotient is not the quotient of the limits when both numerator and denominator approach 0 (resulting in 0/0). Step 8 attempts to fix this by multiplying by x, but the transition from Step 7 to Step 8 is logically incoherent and algebraically invalid as written.
  • gpt-oss:20b: inconclusive 2026-10-06 — reviewer response could not be parsed: {"verdict":"fail","severity":"error","notes":"Step 6 incorrectly divides both numerator and denominator by \(x^2\) instead of \(x\), producing \((8/x-8/x^2)/(4/x-3/x^2)\) which is not algebraically eq

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 2026-10-06 with SymPy 1.14.0.