# 1x3 divided by 3x3

Discussion in 'Homework Help' started by poopscoop, Jan 21, 2014.

1. ### poopscoop Thread Starter Member

Dec 12, 2012
139
16
How do I compute this matrix operation? I havent taken any linear algebra, and I need to learn this for circuit analysis. The instructor recommended Gaussian Elimination, but I feel like there's a much easier way to do this.

What terms do I google? Everything I'm getting isn't matching up with MATLAB computed.

The matrices:
Code ( (Unknown Language)):
1.
2. 5  5  0         I1         50
3. 0  -5 1   X    I2   =    20
4. 1 -1  0         Vc        -2
5.
6.

Feb 19, 2010
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3. ### Papabravo Expert

Feb 24, 2006
10,019
1,756
The MATLAB operator is the backslash '\'

The syntax would be A\b
where a is the 3 x 3 matrix and b is the 1x3 vector of constants
The result is the unknow voltages and currents.

It has the same value as A^(-1)b

4. ### poopscoop Thread Starter Member

Dec 12, 2012
139
16
Unfortunately I have to do this by hand. Anyone here have a preferred method?

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
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I prefer simultaneous equations, it is more writing, but I did not start using matrices until I came to US, so straight up skull work is my first natural response.

Here is how to solve system of simultaneous equations: http://en.wikipedia.org/wiki/System_of_linear_equations#Elimination_of_variables

Last edited: Jan 21, 2014
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Feb 24, 2006
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7. ### poopscoop Thread Starter Member

Dec 12, 2012
139
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I didn't follow it because my guide to Gaussian elimination is poorly written and confusing. It's 2014, this is the only time I will ever do this manually, so I'll spend the extra time and do it with simultaneous equations until someone teaches me Gaussian.

8. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Let us know what you get. I just plugged the system of simultaneous equations that I setup for you in my calculator and already have the answers.

9. ### poopscoop Thread Starter Member

Dec 12, 2012
139
16
We solved it in MATLAB first, so I've had the answers the whole time. Instructor was just making sure we can solve it if we lose battery power during the test. The answers are:

I1: 4
I2: 6
Vc: 50

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,294
482
Yep, same here.

11. ### Papabravo Expert

Feb 24, 2006
10,019
1,756
They are essentially the same thing. It is a distinction without a difference.

12. ### WBahn Moderator

Mar 31, 2012
17,445
4,698
For Pete's sake, take some responsibility for your own education!

If you don't know Gaussian elimination -- then LEARN IT! Teach it to yourself! There are literally hundreds, if not thousands of references on line just waiting for YOU to access them. Until someone teaches it to you -- what a cop out.

If that's an indication of the attitude you are going to take, then do yourself and everyone else a favor and get the hell out of engineering or anything related to it. I've been an engineer for over a quarter century and I still solve systems of simultaneous equations manually on a regular basis.

JoeJester likes this.
13. ### poopscoop Thread Starter Member

Dec 12, 2012
139
16

You are under the impression that because I am in school I'm inexperienced in the workplace and do not take responsibility. I am not some kid, and 25 years of engineering does not give you room to speak down to me. I'm a former special operations operator, hold a 4.0 GPA, and am not to be trifled with. I take more responsibility for my education than anyone you know. I earned my tuition in combat.

I choose not to waste my time teaching myself Gaussian elimination because I will learn it in linear algebra, and for the time being solving the problems manually is an exercise in theory. Its 2014, I will always have access to a calculator and I will always use it.

Now unless you have constructive comments I welcome you to get off your high horse and move along.

14. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,311
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Which calculator do you own that is impervious to a large EMP from a thermo nuclear
device?

15. ### poopscoop Thread Starter Member

Dec 12, 2012
139
16
Well that's why I keep my ti-83 stuffed into my tinfoil underwear.

16. ### MrChips Moderator

Oct 2, 2009
12,227
3,279
Now, now, boys. Learn to get along and play nicely together otherwise we will have to take away your toys and send you to your rooms.

17. ### WBahn Moderator

Mar 31, 2012
17,445
4,698
Am I supposed to be intimidated, somehow? Well, I'm not.

Since you will always have access to a calculator and use it to do your thinking for you, why take linear algebra at all. Just use a calculator program written by someone that was willing to teach themselves things so that you don't have to.

18. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
782
I must confess that whilst I can confidently solve simultaneous equations on paper, I am happy to use whatever tools are at my disposal.

Once I get to three or more equations I go for the easy option. I use Scilab when I have access, and provided I enter the corresponding matrix terms correctly, I can usually obtain an error free solution in short order - which I can verify with the original equations.

19. ### WBahn Moderator

Mar 31, 2012
17,445
4,698
Oh, I have absolutely no problem with that. Use the tools, but as much as possible put forth the effort to understand the tools and at least know how to go about doing it without the tools. Tools are great, when used properly. When latched onto too early they tend to become a crutch and a drug that enables people to think they know how to do something that they really can't. It's a trap that usually comes home to roost, often at a very inconvenient time.

20. ### poopscoop Thread Starter Member

Dec 12, 2012
139
16
Gentleman, keep in mind I spent hundreds f hungry hours learning to use a compass in the age of GPS. I don't need to he lectured on the importance of understanding theory and the old fashioned way of doing things. If you pay attention to the way I ask this and any other question on this forum, you will notice I ask for guidance and not solutions, and want a direction to search.

I am dissapointed in Wbahns comments, and I ask that they be removed, as well as my reply. They reflect a lack of professionalism and an unfounded, disrespectful personal attack that should not be immortalized on these pages.

Barring moderation, if you remove your comments, Wbahn, I will remove mine.