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From: Tato on 29 Jul 2010 04:08
Dear Friends,
Please, help me to solve the following problem. Any hint or comment would be thankful.

I have two expressions:
1) One is Determinant (4X4), which contains two unknown variables (x1,x2).
2) Another is some expression f(x1,x2).

I would like to find solution (x1,x2), which satisfies:
Det=0 and f(x1,x2)=0

Could you please give me some advice. Thank you.

Tato
From: Torsten Hennig on 29 Jul 2010 00:28
> Dear Friends,
> Please, help me to solve the following problem. Any
> hint or comment would be thankful.
>
> I have two expressions:
> 1) One is Determinant (4X4), which contains two
> unknown variables (x1,x2).
> 2) Another is some expression f(x1,x2).
>
> I would like to find solution (x1,x2), which
> satisfies:
> Det=0 and f(x1,x2)=0
>
> Could you please give me some advice. Thank you.
>
> Tato

Is it a standard problem concerned with the
determination of eigenvalues ? Then maybe
MATLAB's eig-function can be of use.
Otherwise you have two equations in two unknowns -
try fsolve.

Best wishes
Torsten.
From: Tato on 29 Jul 2010 05:36
Dear Torsten,

Thank you very much for your reply. But, I am just starting in detail Matlab. I have still some difficulties. Could you give me some additional hint.

For example. I write very simple expressions (mine is much much complicated but just as an example to understand) and please tell me, how can I program in Matlab to solve the unknowns (x1,x2):

for example:

1) Det=[1-x1 1; 6 x2]
2) f(x1,x2)=2*x1-x2

How can I find solutions (x1,x2) satisfying:
Det=0 and f=0.

Thank you very much.
From: Torsten Hennig on 29 Jul 2010 03:11
> Dear Torsten,
>
> Thank you very much for your reply. But, I am just
> starting in detail Matlab. I have still some
> difficulties. Could you give me some additional hint.
>
> For example. I write very simple expressions (mine is
> much much complicated but just as an example to
> understand) and please tell me, how can I program in
> Matlab to solve the unknowns (x1,x2):
>
> for example:
>
> 1) Det=[1-x1 1; 6 x2]
> 2) f(x1,x2)=2*x1-x2
>
> How can I find solutions (x1,x2) satisfying:
> Det=0 and f=0.
>
> Thank you very much.

function F = myfun(x)
F = [(1-x(1))*x(2)-6 ;
2*x(1)-x(2)];

x0 = [-5; -5]; % Make a starting guess at the solution
options=optimset('Display','iter'); % Option to display output
[x,fval] = fsolve(@myfun,x0,options) % Call optimizer

Best wishes
Torsten.
From: Tato on 29 Jul 2010 09:30
Dear Torsten,

Thank you very very much.

Tato
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