Solving Non-Linear Equations

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Aleksandar Vukojevic
评论： Roger Stafford 2014-10-20
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
  1 个评论
Roger Stafford
Roger Stafford 2014-10-20
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.

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采纳的回答

Mohammad Abouali
Mohammad Abouali 2014-10-20
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.

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