Vectors in the plane
Problem 9.275 · easy
For \( \displaystyle \mathbf a = \left\langle -5, 5 \right\rangle \) and \( \displaystyle \mathbf b = \left\langle 2, 0 \right\rangle \), find \( \displaystyle -3\mathbf a + 3\mathbf b \) and its magnitude.
- \[ \left[\begin{matrix}21\\-15\end{matrix}\right] \]Scale each vector, then add componentwise.✓ Proved
- \[ 3 \sqrt{74} \]Its length.✓ Proved
Answer \( \left\langle 21, -15 \right\rangle,\ \text{magnitude}\ 3 \sqrt{74} \)
✓ 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 |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | floating point |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: pass — The solution correctly computes the vector -3a + 3b and its magnitude. The comments accurately describe the operations performed.
Every verdict on record (4)
qwen3.6:27b-mlx: pass 2026-10-05 — The solution correctly computes the vector -3a + 3b and its magnitude. The comments accurately describe the operations performed.gpt-oss:20b: pass 2026-10-05qwen3.6:27b-mlx: pass 2026-10-05 — The solution correctly computes the vector -3a + 3b as <21, -15> and its magnitude as 3*sqrt(74). The steps are algebraically sound and match the stated answer.gpt-oss:20b: pass 2026-10-05
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/vectors_plane, checked 2026-10-05 with SymPy 1.14.0.