Home›Calculus 3›Absolute extrema on a closed region›Problem 10.399

Absolute extrema on a closed region

Problem 10.399 · medium

Find the absolute maximum and minimum values of \( \displaystyle f(x, y) = - 2 x^{2} - x y + 4 x + 2 y^{2} + 2 y \) on the rectangle \( \displaystyle -1 \le x \le 2 \), \( \displaystyle -2 \le y \le 3 \).

\[ \left[\begin{matrix}\frac{\partial}{\partial x} \left(- 2 x^{2} - x y + 4 x + 2 y^{2} + 2 y\right)\\\frac{\partial}{\partial y} \left(- 2 x^{2} - x y + 4 x + 2 y^{2} + 2 y\right)\end{matrix}\right] = \left[\begin{matrix}- 4 x - y + 4\\- x + 4 y + 2\end{matrix}\right] \]
Interior critical points solve ∇f = 0.✓ Proved
On each edge f is a function of one variable: find its critical points there too, and include the four corners.
\[ \left[\begin{matrix}-4\\- \frac{57}{8}\\21\\\frac{193}{8}\\\frac{32}{17}\\\frac{17}{2}\\8\\0\\18\end{matrix}\right] \]
f at every candidate: (-1, -2), (-1, -3/4), (-1, 3), (1/4, 3), (18/17, -4/17), (3/2, -2), (2, -2), (2, 0), (2, 3).✓ Proved
The largest value is 193/8, the smallest -57/8.

Answer \( \max = \frac{193}{8}\ \text{at}\ (\frac{1}{4}, 3);\ \min = - \frac{57}{8}\ \text{at}\ (-1, - \frac{3}{4}) \)

Lines: 2 proved, 2 not checked. 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	simplify(lhs - rhs) reduced to 0
2	Not checked	—	a sentence; read, not computed
3	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
4	Not checked	—	a sentence; read, not computed
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	f sampled on a 241 × 241 grid never beats the claimed max or min, and comes within 5% of both

Reviewers

gpt-oss:20b: fail (misleading) — The solution omits edge critical points (e.g., (1/2,‑2) on y=‑2 and (‑3/4, 3) on y=3). While the final extrema are correct, the statement that all edge critical points were found is false and could mislead a student.
qwen3.6:27b-mlx: pass — The solution correctly identifies the method for finding extrema on a closed region (interior critical points and boundary extrema). The list of candidate points includes the interior critical point, boundary critical points, and corners, and the final comparison of values is correct.

Every verdict on record (4)

gpt-oss:20b: fail (misleading) 2026-10-07 — The solution omits edge critical points (e.g., (1/2,‑2) on y=‑2 and (‑3/4, 3) on y=3). While the final extrema are correct, the statement that all edge critical points were found is false and could mislead a student.
qwen3.6:27b-mlx: pass 2026-10-07 — The solution correctly identifies the method for finding extrema on a closed region (interior critical points and boundary extrema). The list of candidate points includes the interior critical point, boundary critical points, and corners, and the final comparison of values is correct.
qwen3.6:27b-mlx: inconclusive 2026-10-07 — reviewer returned a non-object
gpt-oss:20b: pass 2026-10-07

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 generator:structured/absolute_extrema_2var, checked 2026-10-07 with SymPy 1.14.0.