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Home›Calculus 1›Limits at infinity›Problem 1.115

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 \( \displaystyle \lim_{x \to \infty} \frac{- x + \left(x - 1\right)^{3} + 1}{2 \left(x - 1\right)^{3} + 5} \).
  1. \[ \lim_{x \to \infty}\left(\frac{- x + \left(x - 1\right)^{3} + 1}{2 \left(x - 1\right)^{3} + 5}\right) \]
    limitStart with the limit of the given function.✓ Proved
  2. \[ = \lim_{x \to \infty}\left(\frac{x^{3} - 3 x^{2} + 2 x}{2 x^{3} - 6 x^{2} + 6 x + 3}\right) \]
    algebra simplifyExpand the cubic terms. Simplify the numerator and denominator by combining like terms.✓ Proved
  3. \[ = \lim_{x \to \infty}\left(\frac{1 - \frac{3}{x} + \frac{2}{x^{2}}}{2 - \frac{6}{x} + \frac{6}{x^{2}} + \frac{3}{x^{3}}}\right) \]
    algebra simplifyDivide both numerator and denominator by the highest power of x, which is x**3. Simplify each term in the expression.✓ Proved
  4. \[ = \lim_{x \to \infty} \frac{1}{2} \]
    limit-lawApply the limit laws for each term as x approaches infinity.✓ Proved
  5. \[ = \frac{1}{2} \]
    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
LineStatusChecked byDetail
1✓ Provedsympy 1.14.0line 1 is the problem as stated
2✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where 2*x**3 - 6*x**2 + 6*x + 3 = 0
4✓ Provedsympy 1.14.0simplify(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✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where x = 0
undefined where 2 - 6/x + 6/x**2 + 3/x**3 = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where x = 0
undefined where 2 - 6/x + 6/x**2 + 3/x**3 = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
answer✓ Provedsympy 1.14.0final line against the stated answer: simplify(a - b) reduced to 0
answer, a second way✓ Provedsympy 1.14.0SymPy took the limit from both sides and got the stated value

Reviewers

  • gpt-oss:20b: pass
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • qwen3.6:27b-mlx: pass 2026-10-03
  • gpt-oss:20b: pass 2026-10-03
  • qwen3.6:27b-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.