Originally Posted by

**chrisc** This is an exam question I had. I think I deserve full marks, but the prof is hard to speak too because of his english and I cant seem to find a way to convince him why I deserve full marks. So I want a second opinion, before I go to the department head.

Let A and B be nxn matrices. Can we deduce from AB=B that B = 0 or A = In?

__My answer:__

Let B = 0.

AB = B

A0 = 0

0 = 0

therefore we can deduce that AB=B when B = 0

Let A = In

AB = B

[1,0 - 0,1]B = B

B = B

thefefore we can deduce that AB=B when A = In

(that junk in the squar bracket is an identity matrix)

(for each question, I showed each matrix as a general 2x2 matrix and expanded them to help get my message across)

From what I can understand what his complain was, is that I worked the problem out backwards. If someone can clarify this for me, give me an opinion, please let me know.

I only got 3/6 for the question. However, the exam is only out of 37 so each mark is worth quite a bit.