Related rates
Problem 3.258 · medium
Air is pumped into a spherical balloon at 16 cm³/s. How fast is the radius increasing when the radius is 4 cm?
- The volume of a sphere is V = (4/3)πr³.
- \[ \frac{d}{d r} \frac{4 \pi r^{3}}{3} = 4 \pi r^{2} \]dV/dt = 4πr² dr/dt.✓ Proved
- \[ \left. \frac{4}{\pi r^{2}} \right|_{\substack{ r=4 }} = \frac{1}{4 \pi} \]dr/dt = (dV/dt)/(4πr²) with r = 4.✓ Proved
Answer \( \frac{1}{4 \pi} \ \text{cm/s} \approx 0.07958\ \text{cm/s} \)
Lines: 2 proved, 1 not checked. The answer was also checked a second way, without looking at the solution. A reviewer's objection to the explanation stands; see the receipt.
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 |
| 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 | the situation was stepped forward and back by a microsecond and the quantity differenced numerically |
Reviewers
gpt-oss:20b: fail (error) — The solution omits the given rate dV/dt = 16 cm³/s when computing dr/dt; it substitutes 4/(πr²) instead of 16/(4πr²), leading to an incorrect result.qwen3.6:27b-mlx: fail (error) — The solution fails to explicitly substitute the given rate dV/dt = 16 into the equation. While the final numerical answer is correct, the intermediate step shown in line 3 (Subs(4/(pi*r**2)...)) implies that dV/dt was 4, not 16, or that the division by 16 was skipped in the symbolic representation, making the derivation logically incomplete and potentially confusing.
Every verdict on record (4)
qwen3.6:27b-mlx: fail (error) 2026-09-28 — The solution fails to explicitly substitute the given rate dV/dt = 16 into the equation. While the final numerical answer is correct, the intermediate step shown in line 3 (Subs(4/(pi*r**2)...)) implies that dV/dt was 4, not 16, or that the division by 16 was skipped in the symbolic representation, making the derivation logically incomplete and potentially confusing.gpt-oss:20b: fail (error) 2026-09-28 — The solution omits the given rate dV/dt = 16 cm³/s when computing dr/dt; it substitutes 4/(πr²) instead of 16/(4πr²), leading to an incorrect result.qwen3.6:27b-mlx: fail (error) 2026-09-28 — The solution fails to substitute the given rate of change of volume (dV/dt = 16) into the equation. It incorrectly substitutes 4 for dV/dt in the final calculation, leading to an answer that is off by a factor of 4.gpt-oss:20b: fail (error) 2026-09-28 — The solution omits the given rate dV/dt = 16 cm³/s. Instead it substitutes 4/(π r²), which is not justified. The correct step is dr/dt = (16)/(4πr²) evaluated at r=4, yielding 1/(4π).
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/related_rates, checked 2026-09-28 with SymPy 1.14.0.