multiplying matrices

[sanchez]

Matrices really have me confused. I've read tutorials online, but I still don't get it.

For example, how would I multiply the following 2 matrices.


--- --- --- ---
| 1 2 | | -1 |
| 3 0 | | 5 |
--- --- --- ---

I'm very sorry for the crude depictions, I didn't know any other way to show what I'm trying to solve. Thanks in advance.

 

[stapel]

Matrix addition is done entry-wise. So match up the entries, and add. For instance:


. . . . .\(\displaystyle \large{\left[\begin{array}{ccc}1&2&3\\4&5&6\end{array}\right]\;+\;\left[\begin{array}{rrr}-2&0&1\\3&4&-6\end{array}\right]}\)


. . . . .\(\displaystyle \large{=\;\left[\begin{array}{ccccc}1\,-\,2&\,&2\,+\,0&\,&3\,+\,1\\4\,+\,3&\,&5\,+\,4&\,&6\,-\,6\end{array}\right]}\)


. . . . .\(\displaystyle \large{=\,\left[\begin{array}{rrrrr}-1&\,&2&\,&4\\7&\,&9&\,&0\end{array}\right]}\)


Follow the same procedure with your exercise.

Eliz.

 

[pka]

The real point here is that to add two matrices, both matrices MUST have the same dimensions. Therefore, the two given is the post cannot be added.

 

[sanchez]

Geez, I'm either a total idiot, or I've been up waaay too late working on this assignment.

I read 'product' and thought 'add'. :?

So for those matrices up there I'm actually supposed to be multiplying them, which I also don't know how to do.

 

[stapel]

...for those matrices up there I'm actually supposed to be multiplying them, which I also don't know how to do.

I can't draw a picture for that here, so try this instead. Read the text, and then watch the hand in the picture move from step to step.

(I'm sorry, but I really don't know how to explain matrix multiplication without waving my hands around, which I can't do in this context.)

Hope the link helps at least a little....

Eliz.

 

[sanchez]

I must've sat there staring at that hand animation for 20 minutes before it started to click. But, I think I might have started to understand. It was actually the only example I've seen so far that made any sense to me at all.

In the end I came up with

9
-3
but in one matrix

Does that sound right to you? And I thought the movie the Matrix was confusing, lol.

Thanks for your help and the link.

 

[Gene]

By George, I think you've got it!
---------------
Gene