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Triple integrals in cylindrical coordinates

Problem 11.325 · easy

Use cylindrical coordinates to find the volume of the solid inside \( \displaystyle x^2 + y^2 = 9 \), above \( \displaystyle z = 0 \) and below \( \displaystyle z = 9 - x^2 - y^2 \).

In cylindrical coordinates x² + y² = r² and dV = r dz dr dθ; the region is 0 ≤ θ ≤ 2π, 0 ≤ r ≤ R and z between the surfaces.
\[ \int\limits_{0}^{2 \pi}\int\limits_{0}^{3}\int\limits_{0}^{9 - r^{2}} r\, dz\, dr\, d\theta = \frac{81 \pi}{2} \]
The iterated integral.✓ Proved

Answer \( \frac{81 \pi}{2} \approx 127.23450 \)

Lines: 1 proved, 1 not checked. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation.

The full receipt

Line	Status	Checked by	Detail
1	Not checked	—	a sentence; read, not computed
2	✓ 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	nested quadrature in rectangular coordinates

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: fail (style) — [domain objection, downgraded to style] The setup in line 1 is incomplete because it leaves the radial bound 'R' as an undefined variable rather than specifying R=3 derived from x^2+y^2=9. This makes the logical connection between the problem statement and the integral limits in line 2 ambiguous.

Every verdict on record (4)

gpt-oss:20b: pass 2026-10-09
qwen3.6:27b-mlx: fail (style) 2026-10-09 — [domain objection, downgraded to style] The setup in line 1 is incomplete because it leaves the radial bound 'R' as an undefined variable rather than specifying R=3 derived from x^2+y^2=9. This makes the logical connection between the problem statement and the integral limits in line 2 ambiguous.
gpt-oss:20b: pass 2026-10-09
qwen3.6:27b-mlx: fail (style) 2026-10-09 — [domain objection, downgraded to style] The setup in line 1 is incomplete because it leaves the radial bound 'R' undefined, failing to explicitly derive R=3 from the cylinder equation x^2 + y^2 = 9. While the integral in line 2 uses the correct bound, the solution text does not justify this critical step.

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/cylindrical_integral, checked 2026-10-09 with SymPy 1.14.0.