Electronics Mesh current analysis

Does anyone know how to get the current values from mesh current analysis when you have 3 equations and 3 unknowns, I can do it with 2 using a matrix, finding the determinate etc, but it confuses me a little for 3. I don't think standard simultanious equation methods will work?

some example equations from a question I have just got half way through...

100I₁-60I₂-30I₃=-20
-60I₁+150I₂-40I₃=12
-30I₁-40I₂+90I₃=-10

Thanks
'doc

EDIT: using my calculators symn eqn value finder thingy i get

I₁ = -0.388
I₂ = -0.158
I₃ = -0.31

just doesn't seem right...

 

Hmmm that appears to be correct, must have buggered something else up...

'doc

 

google "cramer's rule" this for instance

 

Yer i got that either when I used it I did something wrong, or it doesn't give me what I'm after.

I learnt a new method 1 hr before my exam seemed to work too, although it took 3 pages

Thanks for the help anyway

'doc

 

Well if you have a TI-89 you can always just use it's solve function. Otherwise, well you could use the 89 pc simulator thing...

But do you not know how to solve a system of 3 equasions and 3 unknowns? Essentially - just use say the first equation to get rid of the first variable in the 2nd and 3rd equastions - say solve the first equasion for one variable, then plug that value into the other two equasions.

 

It was exam so couldn't lug a computer in with me and I could only use a bog standard casio scientific calculator that was provided.

I'm quite aware of that method , I'm not bad on maths (got an A level and doind a maths heavy degree with lots of maths modules) but it doesn't work, thats the method that got me the results in my first post

'doc

 

Hate to say it - but it does work... That's the method I use when my 89 isn't present/allowed. Has yet to fail me.

 

Maybe I missed something, but can't you just solve for one variable (will be in terms of the other two), plug it back into the equations, solve for a second (in terms of the last), plug that back into the third and get a number, and plug THAT in and repeat the process?

And for simplicity's sake, change I1 to A, I2 to B and I3 to C that way your notation won't be as likely to screw you over.

 

It must depend what they are looking for, the paper wouldn't require me to use matracies if I could use a much simpler method. I'm not sure if your talking about mesh current analysis here or just solving simultanious equations.

Firehed, we are allready talking about that and I don't have issues with mixing up variables.

Anyhoo, already done this exam and I appeared to use a method that worked, I know that because I got nice round numbers...

'doc