## Calculus (3rd Edition)

$$\theta=43.89^o.$$
Since $n_1=\langle 1,2,1\rangle$ and $n_2=\langle 4,1,3\rangle$, then we have \begin{align*} \cos \theta &=\frac{n_1 \cdot n_2}{\|n_1\|\|n_2\|}\\ &=\frac{4+2+3}{\sqrt{1+4+1}\sqrt{16+1+9}}\\ &=\frac{9}{\sqrt{6}\sqrt{26}}. \end{align*} That is $$\theta=43.89^o.$$