Points $A$ and $B$ have position vectors $\vec{OA} = \begin{pmatrix}2\\ 2\\ 3\end{pmatrix}$ and $\vec{OB} =\begin{pmatrix}-1\\ 7\\ 2\end{pmatrix}$. Find the angle between $\vec{AB}$ and $\vec{OA}$.
So I found $\vec{AB}=\begin{pmatrix}-3\\ 5\\ -1\end{pmatrix}$.
Then to find the angle, shouldn't I change the sign of the components of the OA vector and then use the formula $\cos \left(\theta \right)=\frac{a\cdot b}{\left|a\right|\left|b\right|}$ ? My textbook mentions that vectors should be pointing away from the angle you are trying to find.