The angle between vectors and is defined as follows.
It's derivation is straightforward.
The angle between two planes is equal to the angle between the planes' normal vectors, and . But then the book says the angle between the two planes is
I assume the absolute value in the numerator is a because we want to take the acute angle between the two lines. Is that right? If not, why is it there? How does the absolute value represent the acute angle?
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