Function writing in MATLAB in symbolic format

I have been trying wrting this equation (screenshot1) in MATLAB in symbolic format. Mathematically, my equation is correct. But it is always giving me error (screeenshot2). As you are seeing the the small piece of code written in screenshot2, can you please help me to write that equation symbolically?

Answer 1

[Paul]

It appears that the symfun objects have to have the same arguments (number and order) to add them

syms rho(x,y,z,t) rho_m(x,y,z,t) rho_p(x,y,z,t)
eq1 = rho(x,y,z,t) == rho_m + rho_p;
disp(eq1)

[Walter Roberson]

Right. The alternative is to invoke the symbolic functions on specific variables (turning them into expressions), add the expressions, and create a symbolic function from the result.