Surface integrals
Problem 12.368 · hard
Evaluate \( \displaystyle \iint_S 1 \, dS \), where \( \displaystyle S \) is the part of the plane \( \displaystyle x + y + z = 4 \) in the first octant.
- Parametrize S by r(u, v) = <u, v, -u - v + 4>; dS = ‖r_u × r_v‖ du dv.Reviewed
- \[ \left(\sqrt{\left(0 \left(-1\right) + 1\right)^{2} + \left(1 - 1 \cdot 0\right)^{2} + \left(0 \cdot 0 + 1 \cdot 1\right)^{2}}\right)^{2} = 3 \]‖r_u × r_v‖² = (sqrt(3))².✓ Proved
- \[ \int\limits_{0}^{4}\int\limits_{0}^{4 - u} \sqrt{3}\, dv\, du = 8 \sqrt{3} \]∬ f(r(u, v)) ‖r_u × r_v‖ du dv.✓ Proved
Answer \( 8 \sqrt{3} \)
✓ Nihil obstat Lines: 2 proved, 1 reviewed. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation. How we verify
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | a sentence; read, not computed |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 3 | ✓ 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 | f times area summed over a triangulated mesh of the surface |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The parametrization correctly models the plane in the first octant, the magnitude of the normal vector is calculated correctly, and the integration bounds and result are accurate.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-17 — The parametrization correctly models the plane in the first octant, the magnitude of the normal vector is calculated correctly, and the integration bounds and result are accurate.gpt-oss:20b: pass 2026-10-17qwen3.6:27b-mlx: pass 2026-10-15 — The parametrization correctly models the plane in the first octant, and the bounds for u and v are correct. The calculation of the surface element and the final integral are correct.gpt-oss:20b: pass 2026-10-15qwen3.6:27b-mlx: pass 2026-10-14 — The parametrization and bounds are correct for the first octant region. The calculation of the surface element magnitude is correct, and the final integration yields the stated answer.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/surface_integral, checked 2026-10-17 with SymPy 1.14.0.