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Home›Calculus 1›L'Hôpital's rule›Problem 1.424

Limit of \( \displaystyle \left(x - 1\right)^{2} e^{1 - x} \) as \( x \to \infty \)

Problem 1.424 · medium

Evaluate \( \displaystyle \lim_{x \to \infty} \left(x - 1\right)^{2} e^{1 - x} \).
  1. \[ \lim_{x \to \infty}\left(\left(x - 1\right)^{2} e^{1 - x}\right) \]
    limit rewriteStart with the original limit. Rewrite the exponential term using negative exponents.✓ Proved
  2. \[ = \lim_{x \to \infty}\left(\left(x^{2} - 2 x + 1\right) e^{1 - x}\right) \]
    algebraExpand the squared binomial.✓ Proved
  3. \[ = \lim_{x \to \infty}\left(\frac{\frac{d}{d x} \left(x^{2} - 2 x + 1\right)}{\frac{d}{d x} e^{x - 1}}\right) \]
    lhopitalApply L'Hopital's rule since the limit is of the form infinity/infinity.✓ Proved
  4. \[ = \lim_{x \to \infty}\left(\left(2 x - 2\right) e^{1 - x}\right) \]
    simplifyCompute the derivatives.✓ Proved
  5. \[ = \lim_{x \to \infty}\left(\frac{\frac{d}{d x} \left(2 x - 2\right)}{\frac{d}{d x} e^{x - 1}}\right) \]
    lhopitalApply L'Hopital's rule again.✓ Proved
  6. \[ = \lim_{x \to \infty}\left(2 e^{1 - x}\right) \]
    simplifyCompute the derivatives.✓ Proved
  7. \[ = \lim_{x \to \infty} 0 \]
    infinityEvaluate the limit as x approaches infinity.✓ Proved
  8. \[ = 0 \]
    limitThe final value of the limit.✓ Proved
Answer \( 0 \)

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
LineStatusChecked byDetail
1✓ Provedsympy 1.14.0line 1 is the problem as stated
2✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
3✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
4✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Derivative(exp(x - 1), x) = 0
5✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Derivative(exp(x - 1), x) = 0
6✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Derivative(exp(x - 1), x) = 0
7✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
undefined where Derivative(exp(x - 1), x) = 0
8✓ Provedsympy 1.14.0simplify(a - b) reduced to 0
9✓ 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: inconclusive — reviewer response could not be parsed: {"verdict":"fail","severity":"error","notes":"Step 4 incorrectly applies L'Hôpital’s rule. The expression \((x-1)^2/\exp(x-1)\) tends to \(0\) as \(x\to\infty\) (finite over infinite), not an \(\infty
  • qwen3.6:27b-mlx: pass
Every verdict on record (4)
  • gpt-oss:20b: inconclusive 2026-10-08 — reviewer response could not be parsed: {"verdict":"fail","severity":"error","notes":"Step 4 incorrectly applies L'Hôpital’s rule. The expression \((x-1)^2/\exp(x-1)\) tends to \(0\) as \(x\to\infty\) (finite over infinite), not an \(\infty
  • qwen3.6:27b-mlx: pass 2026-10-08
  • qwen3.6:27b-mlx: pass 2026-10-08
  • gpt-oss:20b: pass 2026-10-08

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 2026-10-08 with SymPy 1.14.0.