# Homework Help: Zero matrix problem

1. Jan 22, 2006

### DethRose

Ive got a homework question and i dont really understand what is being asked of me

Find a matrix D such that 2a+b-d is the 3x2 zero matrix.

i know a zero matrix is a matrix with all zeros in it so are they trying to get me to make a matrix with all zeros or what?

please help i have been staring at this question for like an hour

2. Jan 23, 2006

### franznietzsche

What are a and b supposed to be? 2a+b should be the same as d if 2a+b-d = 0.

3. Jan 23, 2006

### DethRose

i dont know that all ive got for a question i dont really know where to start

4. Jan 23, 2006

### HallsofIvy

Then the only answer you can give is "d= 2a+ b"!

5. Jan 23, 2006

### DethRose

the answer in the back of the book is

0 6
5 14
-9 9

6. Jan 23, 2006

### shmoe

On it's own, this doesn't make any sense. What are a, b, and d? Are a and b 3x2 matrices you were given earlier? Is little "d" the same as big "D"? There has to more to the question, I suggest you type it out exactly as it appears.

7. Jan 23, 2006

### HallsofIvy

Then a and b must have been given! Read the problem carefully! Perhaps this problem referred to matrices a and b from another problem.

8. Jan 23, 2006

### DethRose

the way i stated it at the beginning is exactly how its put in the textbook

9. Jan 23, 2006

### DethRose

Find a matrix D such that 2a+b-D is the 3x2 zero matrix.

sorry didnt notice i had 2 different d's now it is the exact question

10. Jan 23, 2006

### shmoe

It still makes no sense as written. Again, what are a and b? Were they defined earlier? Is this a multi-part question where a and b were defined in an earlier part? An earlier problem? An example in the text? They have to come from somewhere, or this question and answer is total nonsense.

11. Jan 23, 2006

### DethRose

i just phoned my prof and he said he forgot to put this on the board:

A= 1 3
2 5
-1 2

B= -2 0
1 4
-7 5

12. Jan 23, 2006

### shmoe

Aha!

No problems anymore then?

13. Jan 23, 2006

### DethRose

ive been working on it and im not really sure how to go about doing it?

14. Jan 23, 2006

### shmoe

You now have A and B. You want to find a matrix D where 2A+B-D is the zero matrix. Write D in terms of A and B (Halls did this for you above) and do the arithmetic.