# Matrices question

• Dec 29th 2011, 04:19 AM
Layman
Matrices question
$\begin{bmatrix} 2a+b & -a+b \\ a-b & 3a-2b \end{bmatrix}-\begin{bmatrix} 2a-b & b \\ 4a+b & 2b \end{bmatrix}$

Working through the above problem, an instructional video tells me to solve row 1, column 1 of the matrices as follows;

2a+b - (2a - b)

The video then goes onto used the disributive property, assuming a negative "1" in front of the parentheses, thus "negative 1 multiplied by negative 2a"

Is this correct? The reason I am asking is because I thought it would be negative 1 multipled by positive 2a, as there isn't a sign within the parentheses?

The video I am watching solves the equation as follows;
2a + b - (2a - b)
= 2a + b - 2a + b
= 2a - 2a + b + b
= 2b

I can follow the rest of the solution, I am just confused as to why the teacher assumes a negative sign in front of the 2a which is within the parentheses.
• Dec 29th 2011, 04:23 AM
ILikeSerena
Re: Matrices question
Hi Layman! :)

You are absolutely right.
It should be "negative 1 multipled by positive 2a".
This is also exactly what was done in the solution.
• Dec 29th 2011, 04:28 AM
Layman
Re: Matrices question
Thanks ILikeserena - it seems the instructor made a verbal slip.