Answer
$0.96$ rad
Work Step by Step
The formula to calculate the angle between two planes is:
$ \theta = \cos ^{-1} (\dfrac{p \cdot q}{|p||q|})$
Here, $p=\lt 1,1,1 \gt$ and $q=\lt 0,0,1 \gt$
$|p|=\sqrt{1^2+1^2+1^2}= \sqrt 3$ and $|q|=\sqrt{0^2+0^2+0^2}=1$
Thus, $ \theta = \cos ^{-1} (\dfrac{p \cdot q}{|p||q|})=\cos ^{-1} (\dfrac{1}{ \sqrt 3})$
or, $ \theta =0.96$
