Limit of \( \displaystyle \left(x - 1\right)^{2} e^{1 - x} \) as \( x \to \infty \)
Problem 1.424 · medium
Evaluate \( \displaystyle \lim_{x \to \infty} \left(x - 1\right)^{2} e^{1 - x} \).
- \[ \lim_{x \to \infty}\left(\left(x - 1\right)^{2} e^{1 - x}\right) \]limit rewriteStart with the original limit. Rewrite the exponential term using negative exponents.✓ Proved
- \[ = \lim_{x \to \infty}\left(\left(x^{2} - 2 x + 1\right) e^{1 - x}\right) \]algebraExpand the squared binomial.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{\frac{d}{d x} \left(x^{2} - 2 x + 1\right)}{\frac{d}{d x} e^{x - 1}}\right) \]lhopitalApply L'Hopital's rule since the limit is of the form infinity/infinity.✓ Proved
- \[ = \lim_{x \to \infty}\left(\left(2 x - 2\right) e^{1 - x}\right) \]simplifyCompute the derivatives.✓ Proved
- \[ = \lim_{x \to \infty}\left(\frac{\frac{d}{d x} \left(2 x - 2\right)}{\frac{d}{d x} e^{x - 1}}\right) \]lhopitalApply L'Hopital's rule again.✓ Proved
- \[ = \lim_{x \to \infty}\left(2 e^{1 - x}\right) \]simplifyCompute the derivatives.✓ Proved
- \[ = \lim_{x \to \infty} 0 \]infinityEvaluate the limit as x approaches infinity.✓ Proved
- \[ = 0 \]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
| 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 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Derivative(exp(x - 1), x) = 0 |
| 5 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Derivative(exp(x - 1), x) = 0 |
| 6 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Derivative(exp(x - 1), x) = 0 |
| 7 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 0 undefined where Derivative(exp(x - 1), x) = 0 |
| 8 | ✓ Proved | sympy 1.14.0 | simplify(a - b) reduced to 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: 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 \(\inftyqwen3.6:27b-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 \(\inftyqwen3.6:27b-mlx: pass 2026-10-08qwen3.6:27b-mlx: pass 2026-10-08gpt-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.