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First-order linear equations

Problem 6.101 · medium Video

Solve \( \displaystyle y' + 3y = e^{- 3 x} \) with \( \displaystyle y(0) = -2 \).

The equation is linear in standard form; the integrating factor is e^(∫3 dx) = e^(3x).
Reviewed
\[ \frac{d}{d x} Y{\left(x \right)} e^{3 x} = 3 Y{\left(x \right)} e^{3 x} + e^{3 x} \frac{d}{d x} Y{\left(x \right)} \]
Multiplying by e^(ax) turns the left side into (e^(ax) y)'.✓ Proved
\[ \int 1\, dx = x \]
Integrate the right side.✓ Proved
Setting x = 0 and y = -2 fixes the constant of integration: C = -2.
Reviewed
\[ \left(3 x - 6\right) e^{- 3 x} + \frac{d}{d x} \left(x - 2\right) e^{- 3 x} = e^{- 3 x} \]
The solution satisfies the equation.✓ Proved
\[ -2 \]
And the initial condition.Reviewed

Answer \( y = \left(x - 2\right) e^{- 3 x} \)

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

✓ 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, solves the differential equation, and verifies both the differential equation and the initial condition. The logic is sound.

Every verdict on record (6)

qwen3.6:27b-mlx: pass 2026-09-27 — The solution correctly identifies the integrating factor, solves the differential equation, and verifies both the differential equation and the initial condition. The logic is sound.
gpt-oss:20b: pass 2026-09-27
qwen3.6:27b-mlx: pass 2026-09-26 — The solution correctly identifies the integrating factor, solves the differential equation, and verifies both the differential equation and the initial condition. The logic is sound and the steps are clear.
gpt-oss:20b: pass 2026-09-26
qwen3.6:27b-mlx: pass 2026-09-26 — The solution correctly identifies the integrating factor, solves the differential equation, and determines the constant of integration using the initial condition. The final verification steps confirm the result.
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.