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Riemann sums for double integrals

Problem 11.388 · medium

Estimate \( \displaystyle \iint_R x + y \, dA \) over \( \displaystyle R = [0, 4] \times [0, 2] \) with a Riemann sum, \( \displaystyle m = 2 \) subintervals in \( \displaystyle x \) and \( \displaystyle n = 2 \) in \( \displaystyle y \), sampling at the lower left corners.

\[ 2 \cdot \frac{1}{2} \cdot 4 \cdot \frac{1}{2} = 2 \]
ΔA = Δx Δy.✓ Proved
The sample points: (0, 0), (0, 1), (2, 0), (2, 1).
\[ 2 \cdot 1 \left(0 + 1 + 2 + 3\right) = 12 \]
ΔA times the sum of f at the sample points.✓ Proved
\[ \int\limits_{0}^{2}\int\limits_{0}^{4} \left(x + y\right)\, dx\, dy = 24 \]
For comparison, the exact value is 24.✓ Proved

Answer \( 12 \)

Lines: 3 proved, 1 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. How we verify

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	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	the sum recomputed in exact fractions

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: inconclusive — reviewer returned a non-object

Every verdict on record (6)

qwen3.6:27b-mlx: inconclusive 2026-10-17 — reviewer returned a non-object
gpt-oss:20b: pass 2026-10-17
qwen3.6:27b-mlx: pass 2026-10-15 — The solution correctly identifies the subrectangle area, the lower-left sample points, and computes the Riemann sum accurately. The comparison with the exact integral is also correct.
gpt-oss:20b: pass 2026-10-15
qwen3.6:27b-mlx: fail (error) 2026-10-14 — The sample points listed in step 2 are incorrect for a 2x2 grid with lower-left sampling. The correct points are (0,0), (2,0), (0,1), and (2,1). The solution lists (0,1) twice and omits (2,0), leading to an incorrect sum of function values (6 instead of 8) and a wrong final estimate (12 instead of 16).
gpt-oss:20b: pass 2026-10-14

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/double_riemann, checked 2026-10-17 with SymPy 1.14.0.