## Calculus (3rd Edition)

$$\theta=54.73^o.$$
By making use of the result from problem 36 (d), we get $n_1=\langle 1,1,1\rangle$ and we have $n_2=\langle 1,0,0\rangle$; then we get \begin{align*} \cos \theta &=\frac{n_1 \cdot n_2}{\|n_1\|\|n_2\|}\\ &=\frac{1+0+0}{\sqrt{1+1+1}\sqrt{1+0+0}}\\ &=\frac{1}{\sqrt{3} }. \end{align*} That is $$\theta=54.73^o.$$