A question about the evaluation of a derivative obtained via "syms" command

Hello everyone,

I want to request your help with this exercise in MATLAB. I have started a program in which I need to obtain the derivatives of the equations and then these derivatives would be evaluated in some values of the variables from these derivatives, this is what I got so far...

clc;

clear all;

syms x y

%Input of equations

F=sym(inline('x+x^2*y^2-2*y+3'));
G=sym(inline('y-x^3+2*x*y^2-2'));

%Partial derivatives of equations F & G

Fx=diff(F,x);
Fy=diff(F,y);
Gx=diff(G,x);
Gy=diff(G,y);

%Evaluation of partial derivatives at xi=2 & yi=2

xi=2;
yi=2;

EFX=Fx(xi,yi)
EFY=Fy(xi,yi)
EGx=Gx(xi,yi)
EGy=Gy(xi,yi)

However, after the execution of this program the following message is displayed in the command window:

??? Error using ==> mupadmex
Error in MuPAD command: Index exceeds matrix dimensions.

Error in ==> sym.sym>sym.subsref at 1381
            B = mupadmex('symobj::subsref',A.s,inds{:});

Error in ==> Newton2dimAutomatizado at 21
EFX=Fx(xi,yi)

I am sure that the derivatives are obtained but for some reason these are not evaluated at the point xi=2 & yi=2. So I can´t obtain a value of these derivatives as result of the evaluation at that point.

Can someone help me?

thanks in advance...

Answer 1

[Walter Roberson]

P = sym('some expression');
Px = matlabFunction(diff(P,x));

[Pepe Vázquez]

Thank you