Problem adding two symbolic functions

Hi, I have a problem adding two fuctions with deriatives of symbolic variables. The main purpose ist to solve a differential equation for an amount of x and y to achieve Ux Uz. Can any body help me?

    syms x y Ux Uz
U_x(x,y,Ux)=Ux*cos(x)*cos(y);
U_z(x,y,Uz)=Uz*cos(x)*cos(y);
A1=diff(U_x,x)+0.5*(diff(U_z,x))^2;

Error:

Error using symfun/privResolveArgs (line 181) Symbolic function inputs must match.

Error in sym/privBinaryOp (line 819) args = privResolveArgs(A, B);

Error in + (line 7) X = privBinaryOp(A, B, 'symobj::zip', '_plus');

Answer 1

[Sid Jhaveri]

Hi Hossein,

As the error suggests, both the inputs need to match. Try defining your U_x and U_z as follows:

U_x(x,y,Ux,Uz)=Ux*cos(x)*cos(y);
U_z(x,y,Ux,Uz)=Uz*cos(x)*cos(y);

After changing U_x and U_z as above, your code should work properly.

[Kuifeng Zhao]

Thanks. I came across a similar problem with error "Error in *X = privBinaryOp(A, B, 'symobj::mtimes');" and your answer helped solved my problem.

Answer 2

[Walter Roberson]

A1 = diff(U_x(x,y,Ux),x)+0.5*(diff(U_z(x,y,Uz),x))^2;

That is, diff(U_x,x) gives a function result, as does diff(U_z,x), and then you were trying to add two functions with different arguments. But if you invoke U_x(x,y,Ux) then that becomes an expression result, and you diff() the two expressions and then you can add the expressions.