Solving Non-Linear Equations

I have a problem trying to solve a set of 2 non-linear equations. Here is what I have...

R = (20+0.534*1)/(I^0.88) I = 696/(0.696+R)

I wrote a function for these 2 equations to get them in the form F(x) = 0:

function F = myfun(x) F = [20+0.534*1 - x(1)*(x(2)^0.88); 696-0.696*x(1)-x(1)*x(2)]; end

Then, I wrote:

x0 = [2;3]; options = optimoptions('fsolve','Display','iter'); [x,fval]=fsolve(@myfun,x0,options)

Here is the error:

Error using feval Undefined function 'myfun' for input arguments of type 'double'.

Error in fsolve (line 219) fuser = feval(funfcn{3},x,varargin{:});

Caused by: Failure in initial user-supplied objective function evaluation. FSOLVE cannot continue.

What seems to be the problem here?

Alex

[Roger Stafford]

Your equations and F are not in agreement. In

 20+0.534*1 - x(1)*(x(2)^0.88

x(1) is R and x(2) is I, but in

 696-0.696*x(1)-x(1)*x(2)

x(1) is I and x(2) is R.

Answer 1

[Mohammad Abouali]

Make sure that the following function that you wrote:

function F = myfun(x) 
 F = [20+0.534*1 - x(1)*(x(2)^0.88); 696-0.696*x(1)-x(1)*x(2)]; 
end

is stored in a file called * myfun.m * and it is in the current folder or within the path.

the best to check if it is working or not is to do this, before calling the fsolve

testF=@myfun
testF([2;3])

You should get some numbers. If you got some errors then myfun.m is not accessible.

Another approach is to define your function as follow:

myfun = @(x) [20+0.534*1 - x(1)*(x(2)^0.88); 696-0.696*x(1)-x(1)*x(2)] ;

You can have this before fsolve and it does not require to be stored in a separate m-file.