I'm looking at an example to find the angle between two planes, and I figured you just transfer them into vectors normal to the plane and use the dot product.
I did this (A*B)/(mag(A)mag(B)
The teacher took the absolute value of the dot product, why?
v1 = v2 = <-sqrt(5), 2, 2>
The answer is not cos = (-1/(5sqrt(13)) it is positive. Why?