Integrals of absolute values
Problem 4.747 · easy
Evaluate \( \displaystyle \int_{-2}^{1} \left| - 2 x - 2 \right| dx \).
- \[ - 2 x - 2 \]The integrand is zero at x = -1.✓ Proved
- Between those zeros the sign does not change; where it is negative, |p(x)| = −p(x).Reviewed
- \[ \int\limits_{-2}^{-1} \left(- 2 x - 2\right)\, dx + \int\limits_{-1}^{1} \left(2 x + 2\right)\, dx = 5 \]Integrate each piece and add.✓ Proved
Answer \( 5 \)
Lines: 2 proved, 1 reviewed. 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.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 2 | Reviewed | reviewers: gpt-oss:20b, qwen3.6:27b-mlx | a sentence; read, not computed |
| 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 | numerical quadrature of |p(x)| |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly identifies the root of the integrand, splits the integral at that point, and applies the absolute value definition correctly on each sub-interval. The final calculation is correct.
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-09qwen3.6:27b-mlx: pass 2026-10-09 — The solution correctly identifies the root of the integrand, splits the integral at that point, and applies the absolute value definition correctly on each sub-interval. The final calculation is correct.gpt-oss:20b: inconclusive 2026-10-09 — reviewer returned a non-objectqwen3.6:27b-mlx: fail (error) 2026-10-09 — The solution incorrectly states that the integrand is zero at x = -1 and implies this is the only zero, but fails to explicitly split the integral at this point in the text description before jumping to the calculation. More critically, the sentence 'Between those zeros' is nonsensical as there is only one zero. The setup implies a split at x=-1, but the text is confusing and potentially misleading about the sign change logic.
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/absolute_value_integral, checked 2026-10-09 with SymPy 1.14.0.