Limit of \( \displaystyle \frac{\sqrt{2 x + 1} - 3}{2 x - 8} \) as \( x \to 4 \)
Problem 1.93 · medium
Evaluate \( \displaystyle \lim_{x \to 4} \frac{\sqrt{2 x + 1} - 3}{2 x - 8} \).
- \[ \lim_{x \to 4^+}\left(\frac{\sqrt{2 x + 1} - 3}{2 x - 8}\right) \]limit algebraIdentify the limit to evaluate. Multiply numerator and denominator by the conjugate.✓ Proved
- \[ = \lim_{x \to 4^+} \frac{1}{\sqrt{2 x + 1} + 3} \]algebra simplify cancelExpand the product in the numerator. Simplify the numerator. Cancel the common factor (2*x - 8).✓ Proved
- \[ = \frac{1}{6} \]limit simplify simplify simplifyEvaluate the limit by substituting x = 4. Simplify the square root. Simplify the square root value. Final result.✓ Proved
Answer \( \frac{1}{6} \)
✓ 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
| 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 - 8 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x - 8 = 0 undefined where sqrt(2*x + 1) + 3 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where sqrt(2*x + 1) + 3 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where sqrt(2*x + 1) + 3 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where sqrt(2*x + 1) + 3 = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 9 | ✓ 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 the conjugate method to resolve the indeterminate form. Each step changes only one aspect of the expression and uses appropriate labels from the allowed vocabulary.
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-10-03 — The solution correctly applies the conjugate method to resolve the indeterminate form. Each step changes only one aspect of the expression and uses appropriate labels from the allowed vocabulary.gpt-oss:20b: pass 2026-10-03qwen3.6:27b-mlx: pass 2026-09-30gpt-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.