# Finding Angle Between Planes

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?

$$\cos(\theta) = -cos(\pi - \theta)$$