Vector Scalars
1. **State the problem:** We are given vectors $v = [1, 3]$ and $w = [-2, 5]$ in $\mathbb{R}^2$. We want to find scalars $r$ and $s$ such that $$r v + s w = [-1, 19].$$
2. **Set up the equation:** Writing the vector equation component-wise, we have:
$$r \cdot 1 + s \cdot (-2) = -1$$
$$r \cdot 3 + s \cdot 5 = 19$$
3. **Write the system of linear equations:**
$$\begin{cases} r - 2s = -1 \\ 3r + 5s = 19 \end{cases}$$
4. **Solve the system:** From the first equation, express $r$ in terms of $s$:
$$r = -1 + 2s$$
Substitute into the second equation:
$$3(-1 + 2s) + 5s = 19$$
$$-3 + 6s + 5s = 19$$
$$11s - 3 = 19$$
$$11s = 22$$
$$s = 2$$
5. **Find $r$:** Substitute $s=2$ back into $r = -1 + 2s$:
$$r = -1 + 2 \times 2 = -1 + 4 = 3$$
6. **Check the solution:**
$$r v + s w = 3[1,3] + 2[-2,5] = [3,9] + [-4,10] = [-1,19]$$
This matches the target vector.
**Final answer:**
$$r = 3, \quad s = 2.$$