The derivative from the limit definition
Problem 2.2493 · medium Video
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
- \[ \left. - 3 x^{3} + 2 x \right|_{\substack{ x=h + x }} = - 3 h^{3} - 9 h^{2} x - 9 h x^{2} + 2 h - 3 x^{3} + 2 x \]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
- \[ \lim_{h \to 0^+}\left(- 3 h^{2} - 9 h x - 9 x^{2} + 2\right) = 2 - 9 x^{2} \]Now h → 0 by direct substitution.✓ Proved
- \[ \left. 2 - 9 x^{2} \right|_{\substack{ x=1 }} = -7 \]At x = 1.✓ Proved
Answer \( f'(x) = 2 - 9 x^{2},\ f'(1) = -7 \)
✓ Nihil obstat Lines: 4 proved, 1 reviewed. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation. How we verify
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. The steps are logically sound and the final answer is correct.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-26 — The solution correctly applies the limit definition of the derivative, performs the algebraic simplification accurately, and evaluates the result at x=1. The steps are logically sound and the final answer is correct.gpt-oss:20b: pass 2026-10-26qwen3.6:27b-mlx: pass 2026-10-25gpt-oss:20b: pass 2026-10-25qwen3.6:27b-mlx: pass 2026-10-25 — The solution correctly applies the limit definition of the derivative, performs the algebraic simplification accurately, and evaluates the result correctly.gpt-oss:20b: pass 2026-10-25
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 2026-10-26 with SymPy 1.14.0.