Limits of Riemann sums as integrals
Problem 4.1170 · medium
Write \( \displaystyle \lim_{n\to\infty} \sum_{i=1}^{n} \frac{3 \left(1 + \frac{3 i - 3}{n}\right)^{2} + 3 + \frac{3 \left(3 i - 3\right)}{n}}{n} \) 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) = x**2 + x.
- \[ \int\limits_{1}^{4} \left(x^{2} + x\right)\, dx = \frac{57}{2} \]Evaluate the integral.✓ Proved
- \[ \lim_{n \to \infty} \sum_{i=1}^{n} \frac{3 \left(1 + \frac{3 i - 3}{n}\right)^{2} + 3 + \frac{9 i - 9}{n}}{n} = \frac{57}{2} \]The limit of the sum itself, from the summation formulas, agrees.✓ Proved
Answer \( \int_{1}^{4} x^{2} + x\, dx = \frac{57}{2} \)
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: inconclusive 2026-10-15 — reviewer response could not be parsed: {"verdict":"fail","severity":"error","notes":"The integrand was misidentified: the sum equals \n\[\frac{3x_i^2+3+3x_i}{n}=\frac{3}{n}(x_i^2+x_i+1)=\Delta x\,(x_i^2+x_i+1),\] so the correct integral is
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.