[amelius]

On the first page they say "if neither a or b are 0", but they haven't defined what it means for a vector to be 0.

Also, they say "rank 1 matrix", but they haven't defined the concept of rank yet.

Some readers might find this kind of presentation acceptable, but personally I strongly dislike it when concepts are used before they are defined.

[Koshkin]

To be fair, it does not look like this text is intended to be a self-contained introduction to linear algebra.

[Grustaf]

The concept of an additive zero is built into what it means to be a vector space though.