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Limit of \( \displaystyle \frac{27 x^{3} - 1}{9 x^{2} - 1} \) as \( x \to \frac{1}{3} \)

Problem 1.310 · medium

Evaluate \( \displaystyle \lim_{x \to \frac{1}{3}} \frac{27 x^{3} - 1}{9 x^{2} - 1} \).
  1. \[ \lim_{x \to \frac{1}{3}^+}\left(\frac{27 x^{3} - 1}{9 x^{2} - 1}\right) \]
    limitEvaluate the limit of the given function as x approaches 1/3.✓ Proved
  2. \[ = \lim_{x \to \frac{1}{3}^+}\left(\frac{\left(3 x - 1\right) \left(9 x^{2} + 3 x + 1\right)}{9 x^{2} - 1}\right) \]
    factorFactor the numerator using the difference of cubes formula.✓ Proved
  3. \[ = \lim_{x \to \frac{1}{3}^+}\left(\frac{9 x^{2} + 3 x + 1}{3 x + 1}\right) \]
    factor cancelFactor the denominator using the difference of squares formula. Cancel the common factor (3*x - 1).✓ Proved
  4. \[ = \lim_{x \to \frac{1}{3}^+}\left(9 x^{2} + 3 x + 1\right) \left(\lim_{x \to \frac{1}{3}^+}\left(3 x + 1\right)\right)^{-1} \]
    limit-lawApply the quotient rule for limits.✓ Proved
  5. \[ = \frac{3}{2} \]
    substitution simplify simplifySubstitute x = 1/3 into the expression. Simplify the arithmetic terms. Calculate the final value.✓ Proved
Answer \( \frac{3}{2} \)

✓ 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 9*x**2 - 1 = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 9*x**2 - 1 = 0
undefined where 3*x + 1 = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 3*x + 1 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 3*x + 1 = 0
undefined where Limit(3*x + 1, x, 1/3, dir='+') = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Limit(3*x + 1, x, 1/3, dir='+') = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
8✓ 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
Every verdict on record (4)
  • gpt-oss:20b: pass 2026-10-06
  • qwen3.6:27b-mlx: pass 2026-10-06
  • qwen3.6:27b-mlx: pass 2026-10-06
  • gpt-oss:20b: pass 2026-10-06

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.