# Math Help - Multiplication of Matrices

1. ## Multiplication of Matrices

The question is:

Let A= [2 5, -3 1] and B=[4 -5, 3 k]

I don't know if this will represent A and B better:

[2 , 5] [4 , -5]
[-3 , 1] [3 , k]

What value(s) of k, if any, will make AB=BA?

I got no values of k, but the back of my book claims the answer is (k-5). Can anyone tell me if I am right or wrong?

Thanks,

jarny

2. Originally Posted by jarny
The question is:

Let A= [2 5, -3 1] and B=[4 -5, 3 k]

What value(s) of k, if any, will make AB=BA?

I got no values of k, but the back of my book claims the answer is (k-5). Can anyone tell me if I am right or wrong?

Thanks,

jarny

If you meant to write that your book's answer is $k=5$ then your book's right.

Tonio

3. I got A*B= [23 (5k-10), -9 k+15]
and B*A= [23 15, (6-3k) (k+15)]

Oh okay when I write it on the computer I see I didn't see a negative next to my nine the first time I did it. Thanks tonio

4. Originally Posted by jarny
I got A*B= [23 (5k-10), -9 k+15]
and B*A= [23 15, (6-3k) (k+15)]

Oh okay when I write it on the computer I see I didn't see a negative next to my nine the first time I did it. Thanks tonio

Fine, but a piece of advise: do not do mathematics with your computer! Grab a pencil (or a pen), a sheet of paper (or better: 37436 sheets of paper) and begin to write. Much better if you can get chalk and a blackboard.
After you do maths this way, then you can go to the puter and write it down nicely.

Tonio