4-7 할리데이 10판 솔루션 일반물리학

$$\begin{cases} \vec r_1 = 6.0\i - 7.0\j + 3.0\k\ut{m}\\ \vec r_2 = 3.0\i + 9.0\j - 3.0\k\ut{m}\\ t_{1\to2}=10\ut{s} \end{cases} $$ $$ \begin{aligned} \bar v=&\frac{\Delta\vec r}{\Delta t}\\ =&\frac{\vec r_2-\vec r_1}{\Delta t}\\ =&\frac{(6.0\i - 7.0\j + 3.0\k)-(3.0\i + 9.0\j - 3.0\k)}{10}\\ =&0.3\i-1.0\j+0.6\k\ut{m/s} \end{aligned} $$