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Sums with sigma notation

Problem 4.1037 · easy

Evaluate \( \displaystyle \sum_{i=1}^{15} \left(5 - i\right) \) 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
\[ \left(-1\right) 15 \cdot 16 \cdot 1 \cdot \frac{1}{2} + 5 \cdot 15 = -45 \]
From i = 1 to 15.✓ Proved
\[ \sum_{i=1}^{15} \left(5 - i\right) = -45 \]
The sum, evaluated directly, agrees.✓ Proved

Answer \( -45 \)

✓ 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 applies the linearity of summation and the standard formula for the sum of the first N integers. The arithmetic is correct, and the final result matches the direct evaluation.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-10-17 — The solution correctly applies the linearity of summation and the standard formula for the sum of the first N integers. The arithmetic is correct, and the final result matches the direct evaluation.
gpt-oss:20b: pass 2026-10-17
qwen3.6:27b-mlx: pass 2026-10-14 — The solution correctly applies the standard summation formulas for constants and linear terms. The arithmetic is correct, and the direct evaluation confirms the result.
gpt-oss:20b: pass 2026-10-14
qwen3.6:27b-mlx: pass 2026-10-14 — The solution correctly applies the summation formulas to split the sum into constant and linear parts, and the arithmetic is correct.
gpt-oss:20b: pass 2026-10-14

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 2026-10-17 with SymPy 1.14.0.