## Calculus (3rd Edition)

$$\theta=65.49^o.$$
Since $n_1=\langle 2,3,7\rangle$ and $n_2=\langle 4,-2,2\rangle$, then we have \begin{align*} \cos \theta &=\frac{n_1 \cdot n_2}{\|n_1\|\|n_2\|}\\ &=\frac{8-6+14}{\sqrt{4+9+49}\sqrt{16+4+4}}\\ &=\frac{16}{\sqrt{62}\sqrt{24}}. \end{align*} That is $$\theta=65.49^o.$$