Differentials
Problem 3.437 · easy
Find the differential \( \displaystyle dy \) of \( \displaystyle y = 2 \sqrt{x} \), and evaluate it at \( \displaystyle x = 9 \), \( \displaystyle dx = \frac{1}{20} \).
- \[ \frac{d}{d x} 2 \sqrt{x} = \frac{1}{\sqrt{x}} \]dy = f′(x) dx.✓ Proved
- \[ \frac{\left. 1 \frac{1}{\sqrt{x}} \right|_{\substack{ x=9 }}}{20} = \frac{1}{60} \]Substitute x and dx.✓ Proved
- The exact change is -6 + sqrt(905)/5 ≈ 0.0166436; the differential is the tangent-line estimate of it.Reviewed
Answer \( dy = \frac{1}{\sqrt{x}}\,dx = \frac{1}{60} \)
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 computes the differential and evaluates it. The final sentence provides correct context regarding the relationship between the differential and the exact change, without introducing errors.
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-06qwen3.6:27b-mlx: pass 2026-10-06 — The solution correctly computes the differential and evaluates it. The final sentence provides correct context regarding the relationship between the differential and the exact change, without introducing errors.gpt-oss:20b: fail (misleading) 2026-10-06 — The final sentence claims an exact change of “-6 + sqrt(905)/5”, which is unrelated to the differential calculation and gives a false impression about the problem’s answer. The differential itself is correctly computed as 1/60.qwen3.6:27b-mlx: pass 2026-10-06 — The solution correctly computes the differential and provides accurate context regarding its relationship to the exact change.
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 2026-10-06 with SymPy 1.14.0.