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Area between curves

Problem 5.71 · medium Video

Find the area of the region bounded by \( \displaystyle y = - 3 x^{2} + 13 x - 12 \) and \( \displaystyle y = x - 3 \).

\[ \left(9 - 3 x\right) \left(x - 1\right) = - 3 x^{2} + 12 x - 9 \]
Set the curves equal: top − bottom, factored.✓ Proved
They cross at x = 1 and x = 3; between them y = -3*x**2 + 13*x - 12 is on top.
Reviewed
\[ \int\limits_{1}^{3} \left(- 3 x^{2} + 12 x - 9\right)\, dx = 4 \]
Area = ∫ (top − bottom) dx.✓ Proved

Answer \( 4 \)

The curves y = -3x² + 13x - 12 and y = x - 3 between x = 1 and x = 3. Thin rectangles fill the gap between them, getting narrower until they fill the region exactly; its area is 4.

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. How we verify

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 \|top − bottom\| gives the same area

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly identifies the intersection points, determines the correct integrand (top curve minus bottom curve), and computes the definite integral accurately.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-09-28 — The solution correctly identifies the intersection points, determines the correct integrand (top curve minus bottom curve), and computes the definite integral accurately.
gpt-oss:20b: pass 2026-09-28
qwen3.6:27b-mlx: pass 2026-09-27
gpt-oss:20b: pass 2026-09-27
qwen3.6:27b-mlx: inconclusive 2026-09-26 — reviewer response could not be parsed: { "verdict": "fail", "severity": "error", "notes": "The setup in line 1 is incorrect: the difference between the curves is (-3x^2 + 13x - 12) - (x - 3) = -3x^2 + 12x - 9, but the factored form given,
gpt-oss:20b: pass 2026-09-26

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/area_between_curves, checked 2026-09-28 with SymPy 1.14.0.