substitue part of equation with symbolic value

Hi everybody,

I have an equation in which i should substitue a piece of information (simbolic variables) with a symbolic letter.

to be more clear, I have the following equation

syms x y
Eq=3*x*y+y^2-y^3;

and would like to define A0 so that A0=0.1*x*y so I tried with function subs

modifiedEq1=subs(Eq,(0.1*x*y),A0);
%or
modifiedEq2=subs(3*x*y+y^2-y^3,(0.1*x*y),A0);
%or
modifiedEq3=subs(3*x*y+y^2-y^3==0,(0.1*x*y),A0);

but none of these work and I have no idea even if it is possible to deal with this problem in Matlab.

Thank you everybody for your help.

Answer 1

[Surya Talluri]

I understand that you want to change 0.1*x*y as a in Eq. Since there is no term 0.1*x*y in the Eq, subs function returns the same Eq without any change. You can substitute x*y with 10*a to get the desired result.

Syms a
Eq = subs(Eq, x*y, 10*a)

Please refer through the following resources and examples for further understanding:

• subs - https://www.mathworks.com/help/symbolic/subs.html
• Examples - https://www.mathworks.com/help/symbolic/examples.html?category=symbolic-computations-in-matlab