Laplace transforms from the definition
Problem 4.1105 · medium
Use the definition \( \displaystyle F(s) = \int_0^\infty e^{-st} f(t)\,dt \) to find the Laplace transform of \( \displaystyle f(t) = \cos{\left(t \right)} \), for \( \displaystyle s > 0 \).
- \[ \frac{\partial}{\partial t} \frac{\left(- s \cos{\left(t \right)} + \sin{\left(t \right)}\right) e^{- s t}}{s^{2} + 1} = e^{- s t} \cos{\left(t \right)} \]An antiderivative of e^(−st) f(t), by differentiating it.✓ Proved
- For s > 0 the antiderivative tends to 0 as t → ∞ (the exponential decay wins).Reviewed
- \[ - \left. \frac{\left(- s \cos{\left(t \right)} + \sin{\left(t \right)}\right) e^{- s t}}{s^{2} + 1} \right|_{\substack{ t=0 }} = \frac{s}{s^{2} + 1} \]So the integral is 0 minus its value at t = 0.✓ Proved
Answer \( F(s) = \frac{s}{s^{2} + 1} \)
✓ 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 | ✓ 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 | the defining integral computed by mpmath at two values of s |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly applies the Fundamental Theorem of Calculus using the verified antiderivative. The limit argument at infinity is valid for s > 0, and the evaluation at t=0 yields the correct result.
Every verdict on record (6)
qwen3.6:27b-mlx: pass 2026-10-17 — The solution correctly applies the Fundamental Theorem of Calculus using the verified antiderivative. The limit argument at infinity is valid for s > 0, and the evaluation at t=0 yields the correct result.gpt-oss:20b: pass 2026-10-17qwen3.6:27b-mlx: pass 2026-10-14gpt-oss:20b: pass 2026-10-14qwen3.6:27b-mlx: pass 2026-10-14 — The solution correctly identifies an antiderivative, justifies the boundary condition at infinity for s > 0, and correctly evaluates the definite integral using the Fundamental Theorem of Calculus.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/laplace_transform, checked 2026-10-17 with SymPy 1.14.0.