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First-order linear equations

Problem 6.18 · hard Video

Solve \( \displaystyle y' + 1y = 3 x \) with \( \displaystyle y(0) = -2 \).

The equation is linear in standard form; the integrating factor is e^(∫1 dx) = e^(1x).
Reviewed
\[ \frac{d}{d x} Y{\left(x \right)} e^{x} = Y{\left(x \right)} e^{x} + e^{x} \frac{d}{d x} Y{\left(x \right)} \]
Multiplying by e^(ax) turns the left side into (e^(ax) y)'.✓ Proved
\[ \int 3 x e^{x}\, dx = \left(3 x - 3\right) e^{x} \]
Integrate the right side.✓ Proved
Setting x = 0 and y = -2 fixes the constant of integration: C = 1.
Reviewed
\[ 3 x + \frac{d}{d x} \left(3 x - 3 + e^{- x}\right) - 3 + e^{- x} = 3 x \]
The solution satisfies the equation.✓ Proved
\[ -2 \]
And the initial condition.Reviewed

Answer \( y = 3 x - 3 + e^{- x} \)

The slope field of y' = 3x - y: short segments showing the slope at each point. Several solution curves follow the field; the one through (0, -2) is highlighted, y = 3x - 3 + e^(-x).

✓ Nihil obstat Lines: 3 proved, 3 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
4	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	a sentence; read, not computed
5	✓ Proved	sympy 1.14.0	simplify(lhs - rhs) reduced to 0
6	Reviewed	reviewers: gpt-oss:20b, qwen3.6:27b-mlx	both sides are the same expression: the line states a value, and the working that produced it is not on the line
answer, a second way	✓ Checked independently	sympy 1.14.0 + mpmath 1.3.0	sympy.checkodesol substitutes the solution back; y(0) matches

Reviewers

gpt-oss:20b: pass
qwen3.6:27b-mlx: pass — The solution correctly identifies the integrating factor, performs the integration, and determines the constant of integration. The final answer matches the stated answer.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-09-28 — The solution correctly identifies the integrating factor, performs the integration, and determines the constant of integration. The final answer matches the stated answer.
gpt-oss:20b: pass 2026-09-28
qwen3.6:27b-mlx: pass 2026-09-27 — The solution correctly identifies the integrating factor, performs the integration, determines the constant of integration, and verifies the result. The logic is sound and the steps are clear.
gpt-oss:20b: pass 2026-09-27
qwen3.6:27b-mlx: pass 2026-09-26 — The solution correctly identifies the integrating factor, performs the integration, determines the constant of integration, and verifies the final result against both the differential equation and the initial condition.
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/linear_first_order, checked 2026-10-13 with SymPy 1.14.0.