
Matrices question
$\displaystyle \begin{bmatrix} 2a+b & a+b \\ ab & 3a2b \end{bmatrix}\begin{bmatrix} 2ab & b \\ 4a+b & 2b \end{bmatrix}$
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.

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.

Re: Matrices question
Thanks ILikeserena  it seems the instructor made a verbal slip.