The secant method
Problem 3.720 · medium
Use the secant method on \( \displaystyle f(x) = x^{2} - 2 x - 1 \) with \( \displaystyle x_0 = 2 \) and \( \displaystyle x_1 = 3 \) to find \( \displaystyle x_2 \) and \( \displaystyle x_3 \).
- x_{n+1} = x_n − f(x_n)(x_n − x_{n−1}) / (f(x_n) − f(x_{n−1})).Reviewed
- \[ 3 - \frac{\left. x^{2} - 2 x - 1 \right|_{\substack{ x=3 }}}{- \left. x^{2} - 2 x - 1 \right|_{\substack{ x=2 }} + \left. x^{2} - 2 x - 1 \right|_{\substack{ x=3 }}} = \frac{7}{3} \]x2 ≈ 2.333333.✓ Proved
- \[ \frac{7}{3} + \frac{\left(3 - \frac{7}{3}\right) \left. x^{2} - 2 x - 1 \right|_{\substack{ x=\frac{7}{3} }}}{- \left. x^{2} - 2 x - 1 \right|_{\substack{ x=3 }} + \left. x^{2} - 2 x - 1 \right|_{\substack{ x=\frac{7}{3} }}} = \frac{12}{5} \]x3 ≈ 2.400000.✓ Proved
Answer \( x_2 = \frac{7}{3} \approx 2.333333,\ x_3 = \frac{12}{5} \approx 2.400000 \)
✓ Nihil obstat Lines: 2 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 |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | the iteration run in 30-digit floating point |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly applies the secant method formula and computes the iterations accurately.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-18 — The solution correctly applies the secant method formula and computes the iterations accurately.gpt-oss:20b: pass 2026-10-18qwen3.6:27b-mlx: pass 2026-10-16 — The solution correctly applies the secant method formula and computes the iterations accurately.gpt-oss:20b: pass 2026-10-16qwen3.6:27b-mlx: pass 2026-10-15 — The solution correctly applies the secant method formula and computes the iterations accurately.gpt-oss:20b: pass 2026-10-15
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/secant_method, checked 2026-10-18 with SymPy 1.14.0.