I believe that in my memory this is where it comes from

.

This comes from the angle for vectors.

There, what you do is replace

with

and then what you will have is

.

The interesting part is that since

lies in the same direction of

then

which leads to

beccause

. Note this is for two vectors going in the same direction.

For vectors going in the opposite direction

.

Vectors in this case are parallel and collinear.