Question

Which product is undefined?

Answer 1

AB
BA
CD
DB

Answer 2

When you multiply something like this,
it is undefined.
http://www.mai.liu.se/~hanlu09/matrix/undefined.png

Answer 3

The rows and columns won't match.

Answer 4

So, do they have to match completely?

Answer 5

No.
The ROW of the first matrix has to be of the same size as the COLUMN of the second.

Answer 6

oh okay