Limits of Riemann sums as integrals
Problem 4.1175 · medium
Write \( \displaystyle \lim_{n\to\infty} \sum_{i=1}^{n} \frac{9 + \frac{6 \left(3 i - 3\right)}{n}}{n} \) as a definite integral, and evaluate it.
- Δx = 3/n, and x_i = 1 + 3(i − 1)/n runs across [1, 4] (left endpoints). The sum is Σ f(x_i)Δx with f(x) = 2*x + 1.
- \[ \int\limits_{1}^{4} \left(2 x + 1\right)\, dx = 18 \]Evaluate the integral.✓ Proved
- \[ \lim_{n \to \infty} \sum_{i=1}^{n} \frac{9 + \frac{18 i - 18}{n}}{n} = 18 \]The limit of the sum itself, from the summation formulas, agrees.✓ Proved
Answer \( \int_{1}^{4} 2 x + 1\, dx = 18 \)
Lines: 2 proved, 1 not checked. 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. How we verify
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | Not checked | — | 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 Riemann sum itself with n = 200,000, within its error bound |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: inconclusive — reviewer returned a non-object
Every verdict on record (6)
qwen3.6:27b-mlx: inconclusive 2026-10-18 — reviewer returned a non-objectgpt-oss:20b: pass 2026-10-18qwen3.6:27b-mlx: inconclusive 2026-10-16 — reviewer returned a non-objectgpt-oss:20b: pass 2026-10-16qwen3.6:27b-mlx: inconclusive 2026-10-15 — reviewer returned a non-objectgpt-oss:20b: fail (error) 2026-10-15 — The Riemann sum is set up incorrectly. The chosen Δx=3/n and x_i=1+3(i−1)/n do not produce the summand (9+6(3i−3)/n)/n when written as f(x_i)Δx. Consequently the identified integrand f(x)=2x+1 is wrong, and the evaluation of the integral does not correspond to the original limit.
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/riemann_limit_integral, checked 2026-10-18 with SymPy 1.14.0.