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

Problem 1.324 · medium

Evaluate \( \displaystyle \lim_{x \to -1} \frac{9 x^{2} - 9}{3 x + 3} \).
  1. \[ \lim_{x \to -1^+}\left(\frac{9 x^{2} - 9}{3 x + 3}\right) \]
    limit factorStart with the limit of the given function. Factor the numerator.✓ Proved
  2. \[ = \lim_{x \to -1^+}\left(\frac{\left(x + 1\right) \left(9 x - 9\right)}{3 x + 3}\right) \]
    factor factorFactor the difference of squares. Factor the denominator.✓ Proved
  3. \[ = \lim_{x \to -1^+}\left(3 x - 3\right) \]
    simplifyCancel the common factor (x + 1).✓ Proved
  4. \[ = -6 \]
    limit simplifyEvaluate the expression at x = -1. Final result.✓ Proved
Answer \( -6 \)

✓ 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 3*x + 3 = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 3*x + 3 = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 3*x + 3 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 3*x + 3 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
7✓ 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 factors, cancels, and evaluates the limit. Each step applies a single rule from the allowed vocabulary.
Every verdict on record (4)
  • gpt-oss:20b: pass 2026-10-06
  • qwen3.6:27b-mlx: pass 2026-10-06 — The solution correctly factors, cancels, and evaluates the limit. Each step applies a single rule from the allowed vocabulary.
  • qwen3.6:27b-mlx: pass 2026-10-06 — The solution correctly factors, cancels, and evaluates the limit. Each step applies a single rule and uses valid labels from the vocabulary.
  • 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.