Limit of \( \displaystyle \frac{- x + \left(x - 1\right)^{3} + 1}{2 \left(x - 1\right)^{3} + 5} \) as \( x \to \infty \)
Problem 1.115 · medium
Evaluate \( \displaystyle \lim_{x \to \infty} \frac{- x + \left(x - 1\right)^{3} + 1}{2 \left(x - 1\right)^{3} + 5} \).
- \[ \lim_{x \to \infty}\left(\frac{- x + \left(x - 1\right)^{3} + 1}{2 \left(x - 1\right)^{3} + 5}\right) \]limitStart with the limit of the given function.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{x^{3} - 3 x^{2} + 2 x}{2 x^{3} - 6 x^{2} + 6 x + 3}\right) \]algebra simplifyExpand the cubic terms. Simplify the numerator and denominator by combining like terms.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{1 - \frac{3}{x} + \frac{2}{x^{2}}}{2 - \frac{6}{x} + \frac{6}{x^{2}} + \frac{3}{x^{3}}}\right) \]algebra simplifyDivide both numerator and denominator by the highest power of x, which is x**3. Simplify each term in the expression.✓ Proved
- \[ = \lim_{x \to \infty} \frac{1}{2} \]limit-lawApply the limit laws for each term as x approaches infinity.✓ Proved
- \[ = \frac{1}{2} \]limitEvaluate the final numerical value.✓ Proved
Answer \( \frac{1}{2} \)
✓ 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 - 1)**3 + 5 = 0 undefined where 2*x**3 - 6*x**2 + 6*x + 3 = 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x**3 - 6*x**2 + 6*x + 3 = 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where 2*x**3 - 6*x**2 + 6*x + 3 = 0 undefined where x = 0 undefined where 2 - 6/x + 6/x**2 + 3/x**3 = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where 2 - 6/x + 6/x**2 + 3/x**3 = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where x = 0 undefined where 2 - 6/x + 6/x**2 + 3/x**3 = 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: pass
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-10-03gpt-oss:20b: pass 2026-10-03qwen3.6:27b-mlx: pass 2026-10-03 — The solution correctly expands, simplifies, and evaluates the limit by dividing by the highest power of x. Each step applies a single rule and uses labels from the allowed vocabulary.gpt-oss:20b: pass 2026-10-03
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.