Volumes with known cross sections
Problem 5.542 · medium
The base of a solid is the disk \( \displaystyle x^2 + y^2 \le 4 \). Cross-sections perpendicular to the \( \displaystyle x \)-axis are squares. Find the volume of the solid.
- \[ 2 \sqrt{4 - x^{2}} \]At position x the base of the cross-section has length (top − bottom).✓ Proved
- \[ 16 - 4 x^{2} \]Area of one cross-section: 1·(side)².✓ Proved
- \[ \int\limits_{-2}^{2} \left(16 - 4 x^{2}\right)\, dx = \frac{128}{3} \]Volume = ∫ A(x) dx across the base.✓ Proved
Answer \( \frac{128}{3} \)
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 | simplify(lhs - rhs) reduced to 0 |
| 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 | numerical quadrature of the cross-sectional area |
Reviewers
gpt-oss:20b: fail (error) — The integral of the area 16-4x^2 from -2 to 2 equals 32/3, not 128/3. The solution’s final volume is incorrect.qwen3.6:27b-mlx: pass — The solution correctly identifies the side length of the square cross-sections as the diameter of the disk at position x, calculates the area correctly, and integrates over the correct bounds.
Every verdict on record (4)
gpt-oss:20b: fail (error) 2026-10-13 — The integral of the area 16-4x^2 from -2 to 2 equals 32/3, not 128/3. The solution’s final volume is incorrect.qwen3.6:27b-mlx: pass 2026-10-13 — The solution correctly identifies the side length of the square cross-sections as the diameter of the disk at position x, calculates the area correctly, and integrates over the correct bounds.qwen3.6:27b-mlx: inconclusive 2026-10-13 — reviewer returned a non-objectgpt-oss:20b: pass 2026-10-13
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/volume_cross_sections, checked 2026-10-13 with SymPy 1.14.0.