Calculus of vector-valued functions
Problem 9.420 · medium
Find the unit tangent vector \( \displaystyle \mathbf T \) of \( \displaystyle \mathbf r(t) = \left\langle t^{2}, t^{3}, t \right\rangle \) at \( \displaystyle t = 0 \).
- \[ \left[\begin{matrix}\frac{d}{d t} t^{2}\\\frac{d}{d t} t^{3}\\\frac{d}{d t} t\end{matrix}\right] = \left[\begin{matrix}2 t\\3 t^{2}\\1\end{matrix}\right] \]r′(t).✓ Proved
- \[ 1 \]‖r′(0)‖.✓ Proved
- \[ \left[\begin{matrix}0\\0\\1\end{matrix}\right] \]T = r′/‖r′‖.✓ Proved
Answer \( \mathbf T(0) = \left\langle 0, 0, 1 \right\rangle \)
✓ Nihil obstat Every line of this solution was proved by the computer algebra system SymPy. The answer was also checked a second way, without looking at the solution. Reviewers found nothing wrong with the explanation.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 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 | a numerical velocity, normalised |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly computes the derivative, evaluates its magnitude at t=0, and normalizes the vector. The steps are logically sound and algebraically correct.
Every verdict on record (4)
gpt-oss:20b: pass 2026-10-08qwen3.6:27b-mlx: pass 2026-10-08 — The solution correctly computes the derivative, evaluates its magnitude at t=0, and normalizes the vector. The steps are logically sound and algebraically correct.qwen3.6:27b-mlx: pass 2026-10-08 — The solution correctly computes the derivative, evaluates its magnitude at t=0, and normalizes to find the unit tangent vector. The steps are logically sound and the final answer is correct.gpt-oss:20b: pass 2026-10-08
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/vector_function_calculus, checked 2026-10-08 with SymPy 1.14.0.