dot product

how do you find the angle after doing dot product?

Formula:

$\begin{array}{|rcll|} \hline \vec{a}\cdot \vec{b} &=& a\cdot b \cdot \cos(\varphi) \quad & | \quad |\vec{a}| = a \qquad |\vec{b}| = b \\ \cos(\varphi) &=& \dfrac{ \vec{a}\cdot \vec{b} }{a\cdot b} \\ \hline \end{array}$