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Home›Calculus 1›Limits by factoring›Problem 1.371

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

Problem 1.371 · hard

Evaluate \( \displaystyle \lim_{x \to 1} \frac{- 2 x + \left(2 x - 1\right)^{2} + 1}{\left(2 x - 1\right)^{2} - 1} \).
  1. \[ \lim_{x \to 1^+}\left(\frac{- 2 x + \left(2 x - 1\right)^{2} + 1}{\left(2 x - 1\right)^{2} - 1}\right) \]
    limitEvaluate the limit of the given function at x = 1.✓ Proved
  2. \[ = \lim_{x \to 1^+}\left(\frac{\frac{d}{d x} \left(- 2 x + \left(2 x - 1\right)^{2} + 1\right)}{\frac{d}{d x} \left(\left(2 x - 1\right)^{2} - 1\right)}\right) \]
    lhopitalApply L'Hopital's Rule because the limit is in the form 0/0.✓ Proved
  3. \[ = \lim_{x \to 1^+}\left(\frac{8 x - 6}{8 x - 4}\right) \]
    simplify simplifyCompute the derivatives of the numerator and denominator. Simplify the expression by distributing the constants.✓ Proved
  4. \[ = \lim_{x \to 1^+}\left(1 - \frac{2}{8 x - 4}\right) \]
    algebraSplit the fraction into two parts.✓ Proved
  5. \[ = \lim_{x \to 1^+}\left(1 - \frac{1}{4 x - 2}\right) \]
    simplifySimplify the fraction coefficient.✓ Proved
  6. \[ = \lim_{x \to 1^+} \frac{1}{2} \]
    substitution simplifySubstitute x = 1 into the expression. Evaluate the denominator.✓ Proved
  7. \[ = \frac{1}{2} \]
    limitFinal result.✓ 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 (2*x - 1)**2 - 1 = 0
undefined where Derivative((2*x - 1)**2 - 1, x) = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Derivative((2*x - 1)**2 - 1, x) = 0
undefined where 8*x - 4 = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 8*x - 4 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 8*x - 4 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 8*x - 4 = 0
undefined where 4*x - 2 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 4*x - 2 = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
9✓ 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: fail (style) — Step 3 is labeled "simplify" but it actually applies the derivative rule to both the numerator and the denominator. The label should be "derivative" (or a combined rule if allowed), not "simplify".
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • gpt-oss:20b: fail (style) 2026-10-07 — Step 3 is labeled "simplify" but it actually applies the derivative rule to both the numerator and the denominator. The label should be "derivative" (or a combined rule if allowed), not "simplify".
  • qwen3.6:27b-mlx: pass 2026-10-07
  • gpt-oss:20b: pass 2026-10-07
  • qwen3.6:27b-mlx: pass 2026-10-07 — The solution correctly applies L'Hopital's Rule and performs valid algebraic simplifications. Each step changes only one aspect of the expression and uses an acceptable label from the fixed vocabulary.

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 2026-10-07 with SymPy 1.14.0.