[Scilab-users] Using the Function fsolve

[Robert Sherry]

Samuel,

Thanks for the response. While the example, I showed is linear, the 
equations I want to solve are not. I am thinking I need
a third function which returns a vector. Am I on the right track?

Bob

On 8/28/2016 4:31 PM, Samuel Gougeon wrote:
> Le 28/08/2016 22:03, Robert Sherry a écrit :
>> I defined the following two functions:
>>
>> function [z]=f(x,y)
>> z = x + y - 8
>> endfunction
>>
>> function [z]=g(x,y)
>> z = 2*x + y - 8
>> endfunction
>>
>> I then wanted to find the roots of the two functions (equations). 
>> That is, I want a pair of numbers (a,b)
>> such that f(a,b) = g(a,b) = 0. So, I found the function fsolve in the 
>> documentation of Scilab which I believe will do what I want. So, I 
>> ran the following command:
>>         fsolve([0;0],f,g)
>> and it produced the following error:
>>        Undefined variable: y
>>        at line       2 of function f called by :
>>        fsolve([0;0],f,g)
>> I do not understand this error and I am hoping that somebody can tell 
>> me what I am doing wrong.
>
> You problem is linear. So there are simpler solutions than fsolve() 
> that is useful for systems of non-linear equations.
> Your system is equivalent to the matrix equation
> [1 1 ; 2 1]*X = [ 8 ; 8]  with X = [x ; y]
> Let set
> A = [1 1 ; 2 1] ;
> // and
> b = [ 8 ; 8];
> // from A*X = b, we get X with
> X = A\b
> --> X = A\b
>  X  =
>    0.
>    8.
> that means { x=0, y=8 }
>
> Regards
> Samuel
>
>
>
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