Differentials
Problem 3.491 · easy
The side of a cube changes from \( \displaystyle 9 \) to \( \displaystyle \frac{181}{20} \). Use differentials to estimate the change in its volume, \( \displaystyle dV \).
- \[ \frac{d}{d x} x^{3} = 3 x^{2} \]dy = f′(x) dx.✓ Proved
- \[ \frac{\left. 3 x^{2} \right|_{\substack{ x=9 }}}{20} = \frac{243}{20} \]Substitute x and dx.✓ Proved
- The exact change is 97741/8000 ≈ 12.2176; the differential is the tangent-line estimate of it.Reviewed
Answer \( \frac{243}{20} \)
Lines: 2 proved, 1 reviewed. 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 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | a sentence; read, not computed |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | central difference quotient of f at x times dx |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly identifies the derivative, substitutes the correct values for x and dx, and provides a valid comparison with the exact change.
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-07qwen3.6:27b-mlx: pass 2026-10-07 — The solution correctly identifies the derivative, substitutes the correct values for x and dx, and provides a valid comparison with the exact change.gpt-oss:20b: pass 2026-10-07qwen3.6:27b-mlx: fail (error) 2026-10-07 — The solution fails to explicitly calculate or state the value of dx (which is 0.05 or 1/20) before substitution, making the jump from the derivative formula to the final substitution opaque. Additionally, the final line discusses the 'exact change' but does not explicitly state the estimated change dV as the final answer, which is what the problem asks for.
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/differentials, checked 2026-10-07 with SymPy 1.14.0.