∫Calc Practice

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

Problem 1.62 · medium

Evaluate \( \displaystyle \lim_{x \to 1} \frac{4 x^{2} - 4}{2 x - 2} \).
  1. \[ \lim_{x \to 1^+}\left(\frac{4 x^{2} - 4}{2 x - 2}\right) \]
    limit algebraStart with the limit of the original function. Factor out the constant 4 from the numerator.✓ Proved
  2. \[ = \lim_{x \to 1^+}\left(\frac{\left(x + 1\right) \left(4 x - 4\right)}{2 x - 2}\right) \]
    factor rewriteFactor the difference of squares in the numerator. Factor the denominator.✓ Proved
  3. \[ = \lim_{x \to 1^+}\left(2 x + 2\right) \]
    cancel simplifyCancel the common factor (x - 1). Simplify the fraction.✓ Proved
  4. \[ = 4 \]
    limitEvaluate the limit by substituting x = 1.✓ Proved
Answer \( 4 \)

✓ 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 2*x - 2 = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x - 2 = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x - 2 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x - 2 = 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 applies one rule per step, uses valid labels from the fixed vocabulary, and arrives at the correct result.
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-03 — The solution correctly applies one rule per step, uses valid labels from the fixed vocabulary, and arrives at the correct result.
  • gpt-oss:20b: pass 2026-10-03
  • qwen3.6:27b-mlx: pass 2026-09-30 — The solution correctly applies single rules per step, uses valid labels from the vocabulary, and arrives at the correct limit.
  • gpt-oss:20b: pass 2026-09-30

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 2026-10-03 with SymPy 1.14.0.