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.