FP1 Matrix Help

Matrix A:

First Row: 2, a
Second Row: 0,1


Matrix B
First Row: 2,a
Second Row: 4,1



Shouldnt the Matrix AB be:
First Row: 4+4a, 2a+1
Second Row: 4,1



Because in a Mr M thread he has a different answer

I doubt anyone will be able to comprehend my terrible matrix formating but giving it a go lol

$A_{1,1} B_{1,2} + A_{1,2} B_{2,2} = 2a + 1a = 3a$, not $2a+1$

$\displaystyle AB = \begin{pmatrix}(2)(2)+(a)(4) & (2)(a)+(a)(1) \\ (0)(2)+(1)(4) & (0)(a)+(1)(1) \end{pmatrix} = \begin{pmatrix} 4+4a & 3a \\ 4 & 1 \end{pmatrix}$

"Row by column".

Well, I find that depending on whether you multiply A by B or B by A, you either get
1st row: 4+4a, 3a
2nd row: 4, 1

or

1st row: 4, 3a
2nd row: 8, 4a+1

Thanks for the help. Repped. Must mean Mr M gave the wrong answer then.

Blinkin' cheek!

Edit: I have corrected the thread you are talking about. If you look down to the quote in post 9 you can see that the correct answer was originally posted in the thread but it got messed up when the LaTeX was added later.