Limit of \( \displaystyle \frac{- 3 x + \left(3 x - 1\right)^{3} + 1}{2 \left(3 x - 1\right)^{3} + 5} \) as \( x \to \infty \)
Problem 1.328 · hard
Evaluate \( \displaystyle \lim_{x \to \infty} \frac{- 3 x + \left(3 x - 1\right)^{3} + 1}{2 \left(3 x - 1\right)^{3} + 5} \).
- \[ \lim_{x \to \infty}\left(\frac{- 3 x + \left(3 x - 1\right)^{3} + 1}{2 \left(3 x - 1\right)^{3} + 5}\right) \]limitStart with the limit of the given function as x approaches infinity.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{27 x^{3} - 27 x^{2} + 6 x}{54 x^{3} - 54 x^{2} + 18 x + 3}\right) \]simplifyExpand the binomial terms and simplify the numerator and denominator.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{27 - \frac{27}{x} + \frac{6}{x^{2}}}{54 - \frac{54}{x} + \frac{18}{x^{2}} + \frac{3}{x^{3}}}\right) \]algebra simplifyDivide both the numerator and the denominator by the highest power of x, which is x**3. Simplify the fractions within the limit.✓ Proved
- \[ = \lim_{x \to \infty}\left(27 - \frac{27}{x} + \frac{6}{x^{2}}\right) \left(\lim_{x \to \infty}\left(54 - \frac{54}{x} + \frac{18}{x^{2}} + \frac{3}{x^{3}}\right)\right)^{-1} \]limit-lawApply the quotient rule for limits.✓ Proved
- \[ = \frac{1}{2} \]limit simplifyEvaluate the limits of the numerator and denominator separately as x goes to infinity. Simplify the resulting fraction.✓ Proved
Answer \( \frac{1}{2} \)
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. The reviewers disagree about how one step is explained; every verdict is in the receipt.
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*(3*x - 1)**3 + 5 = 0 undefined where 54*x**3 - 54*x**2 + 18*x + 3 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 54*x**3 - 54*x**2 + 18*x + 3 = 0 undefined where x = 0 undefined where 54 - 54/x + 18/x**2 + 3/x**3 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where 54 - 54/x + 18/x**2 + 3/x**3 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where 54 - 54/x + 18/x**2 + 3/x**3 = 0 undefined where Limit(54 - 54/x + 18/x**2 + 3/x**3, x, oo, dir='-') = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where Limit(54 - 54/x + 18/x**2 + 3/x**3, x, oo, dir='-') = 0 |
| 7 | ✓ 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: fail (error) — Step 2 violates the 'one rule per step' constraint by simultaneously expanding the binomial, combining like terms in the numerator, and simplifying the denominator. Additionally, the note claims to 'expand... and simplify', implying multiple operations were performed in a single step.
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-06qwen3.6:27b-mlx: fail (error) 2026-10-06 — Step 2 violates the 'one rule per step' constraint by simultaneously expanding the binomial, combining like terms in the numerator, and simplifying the denominator. Additionally, the note claims to 'expand... and simplify', implying multiple operations were performed in a single step.qwen3.6:27b-mlx: fail (style) 2026-10-06 — Step 2 applies two distinct operations (expanding the binomial and combining like terms) in a single step, violating the one-change-per-step rule. Step 6 applies the limit evaluation to both the numerator and denominator simultaneously, which should be split into two separate steps.gpt-oss:20b: pass 2026-10-06
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 2026-10-06 with SymPy 1.14.0.