Limit of \( \displaystyle \frac{4 \left(2 x - 1\right)^{2} - 1}{2 x + 2 \left(2 x - 1\right)^{2} - 1} \) as \( x \to \infty \)
Problem 1.65 · hard
Evaluate \( \displaystyle \lim_{x \to \infty} \frac{4 \left(2 x - 1\right)^{2} - 1}{2 x + 2 \left(2 x - 1\right)^{2} - 1} \).
- \[ \lim_{x \to \infty}\left(\frac{4 \left(2 x - 1\right)^{2} - 1}{2 x + 2 \left(2 x - 1\right)^{2} - 1}\right) \]limitStarting with the original limit expression.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{16 x^{2} - 16 x + 3}{8 x^{2} - 6 x + 1}\right) \]algebra algebra simplifyExpand the squared term (2x - 1)^2. Distribute the constants into the parentheses. Combine like terms in the numerator and denominator.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{16 x^{2}}{8 x^{2} - 6 x + 1} - \frac{16 x}{8 x^{2} - 6 x + 1} + \frac{3}{8 x^{2} - 6 x + 1}\right) \]limit-lawSplit the limit into three separate parts using limit laws.✓ Proved
- \[ = - \lim_{x \to \infty}\left(\frac{16 x}{8 x^{2} - 6 x + 1}\right) + \lim_{x \to \infty}\left(\frac{16 x^{2}}{8 x^{2} - 6 x + 1}\right) + \lim_{x \to \infty}\left(\frac{3}{8 x^{2} - 6 x + 1}\right) \]limit-lawApply the limit to each term individually.✓ Proved
- \[ = \lim_{x \to \infty} 2 - \lim_{x \to \infty}\left(\frac{16 x}{8 x^{2} - 6 x + 1}\right) + \lim_{x \to \infty}\left(\frac{3}{8 x^{2} - 6 x + 1}\right) \]simplifyDivide the first term's numerator and denominator by x^2.✓ Proved
- \[ = 2 - \lim_{x \to \infty}\left(\frac{16 x}{8 x^{2} - 6 x + 1}\right) + \lim_{x \to \infty}\left(\frac{3}{8 x^{2} - 6 x + 1}\right) \]simplifyEvaluate the limit of the first term.✓ Proved
- \[ = 2 - \lim_{x \to \infty}\left(\frac{16}{8 x - 6 + \frac{1}{x}}\right) + \lim_{x \to \infty}\left(\frac{3}{8 x^{2} - 6 x + 1}\right) \]algebraDivide the second term's numerator and denominator by x.✓ Proved
- \[ = 2 + \lim_{x \to \infty}\left(\frac{3}{8 x^{2} - 6 x + 1}\right) \]limitEvaluate the limit of the second term as x approaches infinity.✓ Proved
- \[ = 2 \]limit simplifyEvaluate the limit of the third term as x approaches infinity. Final calculation.✓ Proved
Answer \( 2 \)
✓ 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
| 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*(2*x - 1)**2 - 1 = 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 9 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 undefined where 8*x - 6 + 1/x = 0 undefined where x = 0 |
| 10 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x - 6 + 1/x = 0 undefined where 8*x**2 - 6*x + 1 = 0 undefined where x = 0 |
| 11 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 8*x**2 - 6*x + 1 = 0 |
| 12 | ✓ 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 limit laws and algebraic simplifications to evaluate the limit at infinity. 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 2026-10-03 — The solution correctly applies limit laws and algebraic simplifications to evaluate the limit at infinity. Each step adheres to the single-rule constraint and uses valid labels from the fixed vocabulary.gpt-oss:20b: pass 2026-10-03qwen3.6:27b-mlx: pass 2026-09-30 — The solution correctly expands, simplifies, and splits the limit into manageable terms. Each step applies a single rule appropriately, and the logic for evaluating the limits of the individual terms is sound.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.