Limit of \( \displaystyle \frac{4 \left(3 x + 1\right)^{2} - 1}{3 x + 2 \left(3 x + 1\right)^{2} + 1} \) as \( x \to \infty \)
Problem 1.379 · hard
- \[ \lim_{x \to \infty}\left(\frac{4 \left(3 x + 1\right)^{2} - 1}{3 x + 2 \left(3 x + 1\right)^{2} + 1}\right) \]limitStart with the limit of the given function.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{4 \left(3 x + 1\right)^{2} - 1}{3 x + 2 \left(3 x + 1\right)^{2} + 2}\right) \]algebraRearrange the denominator terms.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{36 x^{2} + 24 x + 3}{18 x^{2} + 15 x + 4}\right) \]simplify algebra simplifyExpand the squared binomial term. Distribute the constants into the parentheses. Combine like terms in the numerator and denominator.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{36 + \frac{24}{x} + \frac{3}{x^{2}}}{18 + \frac{15}{x} + \frac{4}{x^{2}}}\right) \]algebra simplifyDivide the numerator and denominator by x**2. Simplify the fractions within the expression.✓ Proved
- \[ = \lim_{x \to \infty} 2 \]limit-lawApply the limit laws for the quotient of limits.✓ Proved
- \[ = 2 \]limitEvaluate the final numerical value.✓ Proved
✓ 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 3*x + 2*(3*x + 1)**2 + 1 = 0 undefined where 3*x + 2*(3*x + 1)**2 + 2 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 3*x + 2*(3*x + 1)**2 + 2 = 0 undefined where 18*x**2 + 15*x + 4 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 18*x**2 + 15*x + 4 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 18*x**2 + 15*x + 4 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 18*x**2 + 15*x + 4 = 0 undefined where x = 0 undefined where 18 + 15/x + 4/x**2 = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where 18 + 15/x + 4/x**2 = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where 18 + 15/x + 4/x**2 = 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 expands, simplifies, and evaluates the limit by dividing by the highest power of x. Each step applies a single rule and is labeled correctly.
Senior review claude-sonnet-5-5, 2026-10-07: pass — All lines are equivalent (SymPy proved each one) and each rule label names a rule that was legitimately applied. The limit-law step collapses the vanishing 1/x^n terms in one move, which is compact but not wrong.
qwen3.6:27b-mlx: dismiss — Step 8 is the standard limit-law step: the 24/x, 3/x², 15/x and 4/x² terms tend to 0 and the quotient law then gives 36/18, so no false statement is made and the label 'limit-law' names the rule actually applied.
Every verdict on record (5)
gpt-oss:20b: pass 2026-10-07qwen3.6:27b-mlx: pass 2026-10-07 — The solution correctly expands, simplifies, and evaluates the limit by dividing by the highest power of x. Each step applies a single rule and is labeled correctly.claude-sonnet-5-5: pass 2026-10-07 — All lines are equivalent (SymPy proved each one) and each rule label names a rule that was legitimately applied. The limit-law step collapses the vanishing 1/x^n terms in one move, which is compact but not wrong.qwen3.6:27b-mlx: fail (error) 2026-10-07 — Step 8 incorrectly applies limit laws by dropping terms that do not vanish (e.g., 24/x vanishes, but the step implies the entire expression reduces to 36/18 without showing the limits of the vanishing terms are 0). A correct application of limit laws would require evaluating the limit of each term separately (e.g., limit of 24/x is 0) or using the 'infinity' rule for rational functions. Simply replacing the expression with 36/18 is a logical leap that skips the evaluation of the limits of the individual terms.gpt-oss:20b: pass 2026-10-07
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.