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Limit of \( \displaystyle \frac{2 x + \left(2 x + 1\right)^{2} - 5}{2 x - 1} \) as \( x \to \frac{1}{2} \)

Problem 1.163 · medium

Evaluate \( \displaystyle \lim_{x \to \frac{1}{2}} \frac{2 x + \left(2 x + 1\right)^{2} - 5}{2 x - 1} \).
  1. \[ \lim_{x \to \frac{1}{2}^+}\left(\frac{2 x + \left(2 x + 1\right)^{2} - 5}{2 x - 1}\right) \]
    limitEvaluate the limit of the given function at x = 1/2.✓ Proved
  2. \[ = \lim_{x \to \frac{1}{2}^+}\left(\frac{4 x^{2} + 6 x - 4}{2 x - 1}\right) \]
    simplify simplify algebraExpand the squared term in the numerator. Combine like terms in the numerator. Split the middle term to prepare for factoring.✓ Proved
  3. \[ = \lim_{x \to \frac{1}{2}^+}\left(2 x + 4\right) \]
    factor cancelFactor the numerator. Cancel the common factor (2*x - 1).✓ Proved
  4. \[ = 5 \]
    limit simplifySubstitute x = 1/2 into the simplified expression. Calculate the final value.✓ Proved
Answer \( 5 \)

✓ 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 = 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x - 1 = 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x - 1 = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x - 1 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 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 — The solution correctly applies algebraic simplification, factoring, and cancellation to resolve the indeterminate form, followed by direct substitution. Each step adheres to the single-rule constraint and uses valid labels from the fixed vocabulary.
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-04 — The solution correctly applies algebraic simplification, factoring, and cancellation to resolve the indeterminate form, followed by direct substitution. Each step adheres to the single-rule constraint and uses valid labels from the fixed vocabulary.
  • gpt-oss:20b: pass 2026-10-04
  • qwen3.6:27b-mlx: pass 2026-10-04 — The solution correctly expands, factors, and cancels the indeterminate form. Each step applies a single rule from the allowed vocabulary, and the logic is sound.
  • gpt-oss:20b: pass 2026-10-04

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 2026-10-04 with SymPy 1.14.0.