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# substitue part of equation with symbolic value

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gabriele fadanelli on 4 Aug 2020
Answered: Surya Talluri on 7 Aug 2020
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.
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### Answers (1)

Surya Talluri on 7 Aug 2020
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:
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