Maths Matrix Question

[elster]

I have a slight maths problem that I think I am going wrong somewhere.

A=

2	1
1	3

B=

-1	3a
-4b	3

AB=

2	9
11	12

This looks quite simple. Going along the rows of A and multiplying with the columns of B.

Getting this

(2 x -1)+(-4bx1)	(2x3a)+(1x3)
(1x-1)+(1x-4b)	(1x3a)+(3x3)

But this can't be right, as a=1 which is fine. But b is coming out as -1 and -5/2.

This can't be so as they both have to be the same number.

Can anyone see what I am doing wrong?

[blinkythefish]

Bottom left corner should be 3x-4b not 1

Edited by blinkythefish on Wednesday 24th February 18:50

[elster]

blinkythefish said:

Bottom left corner should be 3x-4b not 1

Edited by blinkythefish on Wednesday 24th February 18:50

Ah superb. I think staring at it all and writing out several times has sent my brain to mush.

Thank you.