Answer
$\dfrac{\pi}{2}$ or $90 ^{\circ}$
Work Step by Step
The formula to calculate the angle between two planes is:
$ \theta = \cos ^{-1} (\dfrac{p \cdot q}{|p||q|})$
Here, $\overrightarrow{AC}=\lt 2,4 \gt$ and $\overrightarrow{DB}=\lt -4,2 \gt$
$|\overrightarrow{AC}|=\sqrt{(2)^2+(4)^2}= \sqrt {20}$ and $|\overrightarrow{DB}|=\sqrt{(-4)^2+(2)^2}=\sqrt {20}$
Thus, $ \theta = \cos ^{-1} (\dfrac{\overrightarrow{AC} \cdot \overrightarrow{DB}}{|\overrightarrow{AC}||\overrightarrow{DB}|})=\cos ^{-1} (\dfrac{(2)(-4)+(4)(2)}{ ( \sqrt {20})(\sqrt 20)})=\cos ^{-1} (0)$
or, $ \theta =\dfrac{\pi}{2}$ or $90 ^{\circ}$
