Answer
$1.55$ or $\approx 88.88 ^{\circ}$
Work Step by Step
The formula to calculate the angle between two planes is:
$ \theta = \cos ^{-1} (\dfrac{p \cdot q}{|p||q|})$
Here, $u=\lt 0,1,0.2 \gt$ and $v=\lt 1,0,0.1 \gt$
$|u|=\sqrt{(0)^2+(1)^2+(0.2)^2}= \sqrt {1.04}$ and $|v|=\sqrt{(1)^2+(0)^2+(0.1)^2}=\sqrt {1.01}$
Thus, $ \theta = \cos ^{-1} (\dfrac{u \cdot v}{|u||v|})=\cos ^{-1} (\dfrac{0.2}{ ( \sqrt {1.04})(\sqrt {1.01}})$
or, $ \theta \approx 1.55$ or $\approx 88.88 ^{\circ}$
