A good answer might be:

When one of the vectors is a zero vector:

length(u + 0) <= length(u) + length(0)

Diagram for Adding the Zero Vector


Add the zero matrix ( 0, 0 )T to the matrix ( 4, 3 )T. If (0, 0)T represents a displacement vector, it means do nothing to change position. In the diagram, no arrow can be drawn for the zero vector since it has no length.


Now consider what this might mean: add the vector (1, 2)T to the point (4, 4)T.

If you think that makes sense, then do the calculation. What type of object is the result?