matlab nonlinear equation solver

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myetceteramail myetceteramail
评论： Matt J 2017-12-10
i have a set of3 nonlinear equations and i need to solve them by using fsolve in matlab
function F = root2d(y)
syms b1 b2 b3 w21 w31 theta1 theta2 theta3 a1 a2 a3;
F(1) = (1+exp(-b1*(w21*y(2)+w31*y(3)-theta1)))^(-1) - a1*y(1);
F(2) = (1/(1+exp(-b2*(y(1)-theta2))))-a2*y(2);
F(3)= (1/(1+exp(-b3*(y(1)-theta3))))-a3*y(3);
this is my function matlab file. I call this matlab function while using the matlab file
fun = @root2d;
x0 = [0,0,0];
x = fsolve(fun,x0)
But it is giving me the error
Error using fsolve (line 258)
FSOLVE requires all values returned by user functions to be of data type double.
Error in Untitled5 (line 3)
x = fsolve(fun,x0)
can somebody help ?

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回答（1 个）

Matt J
Matt J 2017-12-10
编辑：Matt J 2017-12-10
FSOLVE does not handle symbolic variables, so get rid of the line
syms b1 b2 b3 w21 w31 theta1 theta2 theta3 a1 a2 a3
and substitute these quantities with actual numeric values.
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myetceteramail myetceteramail
myetceteramail myetceteramail 2017-12-10
function F = root2d(y)
syms b1 b2 b3 w21 w31 theta1 theta2 theta3 a1 a2 a3;
F(1) = (1+exp(-b1*(w21*y(2)+w31*y(3)-theta1)))^(-1) - a1*y(1);
F(2) = (1/(1+exp(-b2*(y(1)-theta2))))-a2*y(2);
F(3)= (1/(1+exp(-b3*(y(1)-theta3))))-a3*y(3);
fun = @root2d;
x0 = [0,0,0];
x = solve(fun,x0);
Matt J
Matt J 2017-12-10
To clarify what I said before, you must use the SOLVE command, but you should first read its documentation so you can use it correctly!

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