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Limit of \( \displaystyle \frac{27 x^{3} - 3 x}{54 x^{3} + 5} \) as \( x \to \infty \)

Problem 1.154 · medium

Evaluate \( \displaystyle \lim_{x \to \infty} \frac{27 x^{3} - 3 x}{54 x^{3} + 5} \).
  1. \[ \lim_{x \to \infty}\left(\frac{27 x^{3} - 3 x}{54 x^{3} + 5}\right) \]
    limit algebraIdentify the limit to be evaluated. Divide both numerator and denominator by x**3.✓ Proved
  2. \[ = \lim_{x \to \infty}\left(\frac{27 - \frac{3}{x^{2}}}{54 + \frac{5}{x^{3}}}\right) \]
    simplifySimplify the terms inside the limit.✓ Proved
  3. \[ = \lim_{x \to \infty}\left(27 - \frac{3}{x^{2}}\right) \left(\lim_{x \to \infty}\left(54 + \frac{5}{x^{3}}\right)\right)^{-1} \]
    limit-lawApply the quotient rule for limits.✓ Proved
  4. \[ = \frac{1}{2} \]
    limit simplifyEvaluate the limits of the numerator and denominator separately. Reduce the fraction to its simplest form.✓ Proved
Answer \( \frac{1}{2} \)

✓ 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
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 54*x**3 + 5 = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 54*x**3 + 5 = 0
undefined where 54 + 5/x**3 = 0
undefined where x = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 54 + 5/x**3 = 0
undefined where x = 0
undefined where Limit(54 + 5/x**3, x, oo, dir='-') = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Limit(54 + 5/x**3, x, oo, dir='-') = 0
undefined where x = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 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: pass — The solution correctly applies algebraic manipulation to normalize the rational function, uses the quotient law for limits, and evaluates the resulting constant limits. Each step changes only one aspect of the expression and uses valid labels from the vocabulary.
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-04 — The solution correctly applies algebraic manipulation to normalize the rational function, uses the quotient law for limits, and evaluates the resulting constant limits. Each step changes only one aspect of the expression and uses valid labels from the vocabulary.
  • gpt-oss:20b: pass 2026-10-04
  • qwen3.6:27b-mlx: pass 2026-10-04
  • gpt-oss:20b: pass 2026-10-04

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 2026-10-04 with SymPy 1.14.0.