The derivative from the limit definition
Problem 2.1792 · medium
Use the limit definition of the derivative to find \( \displaystyle f'(x) \) for \( \displaystyle f(x) = 3 x^{3} + 2 x \), then find \( \displaystyle f'(1) \).
- By definition f′(x) = lim_{h→0} [f(x + h) − f(x)]/h.Reviewed
- \[ 2 h + 2 x + 3 \left(h + x\right)^{3} = 3 h^{3} + 9 h^{2} x + 9 h x^{2} + 2 h + 3 x^{3} + 2 x \]Write out f(x + h).✓ Proved
- \[ \frac{2 h - 3 x^{3} + 3 \left(h + x\right)^{3}}{h} = 3 h^{2} + 9 h x + 9 x^{2} + 2 \]Combine and cancel the factor h.✓ Proved
- \[ \lim_{h \to 0^+}\left(3 h^{2} + 9 h x + 9 x^{2} + 2\right) = 9 x^{2} + 2 \]Now h → 0 by direct substitution.✓ Proved
- \[ \left. 9 x^{2} + 2 \right|_{\substack{ x=1 }} = 11 \]At x = 1.✓ Proved
Answer \( f'(x) = 9 x^{2} + 2,\ f'(1) = 11 \)
Lines: 4 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 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | 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 |
| 4 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 5 | ✓ 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 | matches the derivative by the power, quotient and chain rules |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly applies the limit definition of the derivative, performs the algebraic simplification accurately, and evaluates the result at x=1.
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-07qwen3.6:27b-mlx: pass 2026-10-07 — The solution correctly applies the limit definition of the derivative, performs the algebraic simplification accurately, and evaluates the result at x=1.gpt-oss:20b: fail (error) 2026-10-07 — The third sentence incorrectly forms the difference f(x+h)-f(x); it subtracts 3x^3 but omits the 2x term, leading to an incorrect numerator and thus an incorrect derivative.qwen3.6:27b-mlx: pass 2026-10-07 — The solution correctly applies the limit definition of the derivative, performs the algebraic simplification accurately, and evaluates the result at x=1.
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/derivative_definition, checked 2026-10-07 with SymPy 1.14.0.