3D vectors
Same as 2D (directed line segments with position
ignored), but we have different properties.
In 2D, the vector perpendicular to a given vector
is unique (up to a scale).
Two 3D vectors in 3D can be multiplied to get a vector (vector or cross product).
Dot product works the same way, but the coordinate expression is
(v,w) = vxwx+ vywy + vzwz