Thomas' Calculus 13th Edition

$\approx 1.55$ or $\approx 88.88 ^{\circ}$
The angle between two planes can be determined as: $\theta = \cos ^{-1} (\dfrac{a \cdot b}{|a||b|})$ Now, $a=\lt 0,1,0.2 \gt$ and $b=\lt 1,0,0.1 \gt$ Here, $|a|=\sqrt{(0)^2+(1)^2+(0.2)^2}= \sqrt {1.04}$; $|b|=\sqrt{(1)^2+(0)^2+(0.1)^2}=\sqrt {1.01}$ So, $\theta =\cos ^{-1} (\dfrac{0.2}{ ( \sqrt {1.04})(\sqrt {1.01}}) \approx 1.55$ or $\approx 88.88 ^{\circ}$