# How can I solve an equation in a While loop?

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mr mo on 12 Apr 2018
Commented: Walter Roberson on 16 Apr 2018
Hi. I have a equation like below.
[m1 m2 ; m3 m4] * [d1 ; d2] = [c1 ; c2]
The values of m1, m2, m3, m4 and c1, c2 change in a while loop.
I want to find the d1 and d2 values in every Iteration of the mentioned while loop.
I wrote this, but some times it return Nan values.
[d1 ; d2] = [m1 m2 ; m3 m4] \ [c1 ; c2];
How can I do that?
Thanks a lot.

Walter Roberson on 12 Apr 2018
That equation does not always have solutions. Your m might be singular or the calculation might overflow or might contain inf in ways lead to nan.
If you have the symbolic toolbox sometimes you can get further by putting sym() around the two sides of the \ operator, but if that helps you probably need to go back and do the calculation of the m and c values as sym.
Walter Roberson on 16 Apr 2018
Please follow the debugging instructions I indicated with dbstop and so on.

Torsten on 12 Apr 2018
Maybe m1, m2, m3, m4, c1, c2 are NaN ?
And use
D = [m1 m2 ; m3 m4] \ [c1 ; c2];
d1 = D(1);
d2 = D(2);
And always make sure that det(M) does not equal 0 before computing D.
Best wishes
Torsten.