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 $\dfrac{\vec{A}\cdot \vec{B}}{|\vec{A}||\vec{B}|}$.
The teacher took the absolute value of the dot product, why?
$v_1 = \langle\sqrt{(5)}, 4, -2\rangle.$
$v_2 = \langle -\sqrt{(5)}, 2, 2\rangle.$
The answer is not $\cos ? = \dfrac{-1}{5\sqrt{(13)}}$ it is positive.
Why not?