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Sums with sigma notation

Problem 4.1468 · easy

Evaluate \( \displaystyle \sum_{i=1}^{20} \left(3 i^{2} - 3 i - 2\right) \) using summation formulas.

Split the sum and use Σᵢ₌₁ᴺ 1 = N, Σ i = N(N + 1)/2, Σ i² = N(N + 1)(2N + 1)/6, Σ i³ = [N(N + 1)/2]².
Reviewed
\[ \left(-3\right) 20 \cdot 21 \cdot 1 \cdot \frac{1}{2} - 2 \cdot 20 + 3 \cdot 20 \cdot 21 \cdot 41 \cdot 1 \cdot \frac{1}{6} = 7940 \]
From i = 1 to 20.✓ Proved
\[ \sum_{i=1}^{20} \left(3 i^{2} - 3 i - 2\right) = 7940 \]
The sum, evaluated directly, agrees.✓ Proved

Answer \( 7940 \)

✓ 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 terms added one by one in Python integers

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly identifies the summation formulas and applies them to the specific terms. The algebraic verification confirms the stated answer is correct.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-11-08 — The solution correctly identifies the summation formulas and applies them to the specific terms. The algebraic verification confirms the stated answer is correct.
gpt-oss:20b: pass 2026-11-08
qwen3.6:27b-mlx: pass 2026-11-07 — The solution correctly applies standard summation formulas and verifies the result. The inclusion of the cubic sum formula in the setup is unnecessary but harmless.
gpt-oss:20b: pass 2026-11-07
qwen3.6:27b-mlx: pass 2026-11-07 — The solution correctly identifies the summation formulas needed and applies them to the specific bounds. The arithmetic in the unchecked equation is correct, and the direct evaluation confirms the result.
gpt-oss:20b: pass 2026-11-07

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/summation_formulas, checked 2026-11-08 with SymPy 1.14.0.