Limit of \( \displaystyle \frac{4 x^{2} - 4}{2 x - 2} \) as \( x \to 1 \)
Problem 1.62 · medium
Evaluate \( \displaystyle \lim_{x \to 1} \frac{4 x^{2} - 4}{2 x - 2} \).
- \[ \lim_{x \to 1^+}\left(\frac{4 x^{2} - 4}{2 x - 2}\right) \]limit algebraStart with the limit of the original function. Factor out the constant 4 from the numerator.✓ Proved
- \[ = \lim_{x \to 1^+}\left(\frac{\left(x + 1\right) \left(4 x - 4\right)}{2 x - 2}\right) \]factor rewriteFactor the difference of squares in the numerator. Factor the denominator.✓ Proved
- \[ = \lim_{x \to 1^+}\left(2 x + 2\right) \]cancel simplifyCancel the common factor (x - 1). Simplify the fraction.✓ Proved
- \[ = 4 \]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
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | line 1 is the problem as stated |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x - 2 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x - 2 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x - 2 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x - 2 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| answer | ✓ Proved | sympy 1.14.0 | final line against the stated answer: simplify(a - b) reduced to 0 |
| answer, a second way | ✓ Proved | sympy 1.14.0 | SymPy took the limit from both sides and got the stated value |
Reviewers
gpt-oss:20b: passqwen3.6:27b-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 2026-10-03qwen3.6:27b-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.