問題 10

Given vectors $\vec{a} = 2\hat{i} + 3\hat{j}$ and $\vec{b} = \hat{i} - 2\hat{j}$, what is the scalar projection of $\vec{a}$ onto $\vec{b}$?

1. $-\frac{4}{\sqrt{5}}$
2. $\frac{4}{\sqrt{5}}$
3. $-4$
4. $\frac{2}{\sqrt{13}}$