Answer
$0.75$ 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, $p=\lt 2,1,0 \gt$ and $q=\lt 1,2,-1 \gt$
$|p|=\sqrt{2^2+1^2+0^2}= \sqrt {5}$ and $|q|=\sqrt{1^2+2^2+(-1)^2}=\sqrt 6$
Thus, $ \theta = \cos ^{-1} (\dfrac{p \cdot q}{|p||q|})=\cos ^{-1} (\dfrac{4}{ (\sqrt 5)(\sqrt 6)})$
or, $ \theta \approx 0.75$ rad
