Surface area of revolution
Problem 5.246 · medium
Find the area of the surface obtained by rotating \( \displaystyle y = \sqrt{x} \), \( \displaystyle 3 \le x \le 6 \), about the \( \displaystyle x \)-axis.
- \[ \frac{d}{d x} \sqrt{x} = \frac{1}{2 \sqrt{x}} \]y′.✓ Proved
- Rotating about the x-axis, the radius is y, so S = ∫ 2π·(radius)·√(1 + (y′)²) dx.
- \[ 2 \pi \sqrt{x} \sqrt{1 + \frac{1}{4 x}} = \pi \sqrt{x} \sqrt{4 + \frac{1}{x}} \]Simplify the integrand.✓ Proved
- \[ \int\limits_{3}^{6} \pi \sqrt{x} \sqrt{4 + \frac{1}{x}}\, dx = \frac{\pi \left(125 - 13 \sqrt{13}\right)}{6} \]Integrate.✓ Proved
Answer \( \frac{\pi \left(125 - 13 \sqrt{13}\right)}{6} \approx 40.9076 \)
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.
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 | numerical quadrature of the unsimplified integrand |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: fail (error) — The simplification in line 3 is algebraically incorrect. The expression 2*pi*sqrt(x)*sqrt(1 + 1/(4*x)) simplifies to pi*sqrt(4x + 1), not pi*sqrt(x)*sqrt(4 + 1/x). Consequently, the integral in line 4 is set up with the wrong integrand, leading to an incorrect final answer.
Every verdict on record (4)
qwen3.6:27b-mlx: fail (error) 2026-10-05 — The simplification in line 3 is algebraically incorrect. The expression 2*pi*sqrt(x)*sqrt(1 + 1/(4*x)) simplifies to pi*sqrt(4x + 1), not pi*sqrt(x)*sqrt(4 + 1/x). Consequently, the integral in line 4 is set up with the wrong integrand, leading to an incorrect final answer.gpt-oss:20b: pass 2026-10-05qwen3.6:27b-mlx: fail (error) 2026-10-05 — The simplification in step 3 is algebraically incorrect. The expression 2*pi*sqrt(x)*sqrt(1 + 1/(4*x)) simplifies to pi*sqrt(4x + 1), not pi*sqrt(x)*sqrt(4 + 1/x). Consequently, the integration in step 4 is performed on the wrong function, leading to an incorrect final answer.gpt-oss:20b: pass 2026-10-05
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_area_revolution, checked 2026-10-05 with SymPy 1.14.0.