1. ## Adding and Subtracting Matrices

I've got this problem from my SAT Math level 2 prep book, which I think has the wrong answer.

The question is:

So what I did was subtract the matrix on the right by the left, and then both multiply both sides by -1.

(-4 1 -11)
8 -1 -2

However in the book, the answer says b)

I have never done matrices before reading this book, and I'm trying to learn from the book. This is proving difficult as this book is quite hard to read, although I correctly answered the previous questions.

2. ## Re: Adding and Subtracting Matrices

Hello, fobbz!

Solve for $\displaystyle X:\;\;\begin{pmatrix} 1&2&3 \\ 2&1&3\end{pmatrix} - X \;=\;\begin{pmatrix}5&1&8 \\ \text{-}6&0&5\end{pmatrix}$

We have: .$\displaystyle X \;=\;\begin{pmatrix} 1&2&3 \\ 2&1&3\end{pmatrix} - \begin{pmatrix} 5&1&8\\ \text{-}6&0&5\end{pmatrix}$

Therefore: .$\displaystyle X \;=\;\begin{pmatrix}\text{-}4 & 1 & \text{-}5 \\ 8 & 1 & \text{-}2\end{pmatrix}$

3. ## Re: Adding and Subtracting Matrices

because matrix arithmetic involves a lot of arithmetic operations "all at once", it is not uncommon to make various sign errors or other simple arithmetic mistakes in the process of doing one's calculations. i should know, this happens to me all the time.

so one must be extra vigilant in checking and re-checking your work.

it seems clear that what happened, is you added 8, instead of subtracting it (in the upper-right hand corner), other than that, everything is fine. your logic was fine, there was an error in execution (a sign error).