How to replace equations in symbolic function

Good Morning,

Please I have a function F = 3*a*b

Therefore how can i change (a*b) with a variable 'x' to obtain F = 3*x

Think you.

Answer 1

[Birdman]

By using subs function:

syms a b x
F=3*a*b;
Fnew=subs(F,{a,b},{x,1})

[Anis Frej]

Think you very much Bridman but don't work with my problem

beacause my problem that I have a function F = A*(q1^2) + B*(q2^2) + C*(q1*q2) + D*(q2*q3);

And I want change : (q1^2) with t1 / (q2^2) with t2 / (q1*q2) with t12 / (q2*q3) with t23

to obtain F = A*t1 + B*t2 + C*t12 + D*t23

Think you.

[Birdman]

That 1 stands for replacing with b.

Your problem should be solved by the following code:

syms A B C D q1 q2 q3 t1 t2 t12 t23
F=A*q1^2+B*q2^2+C*q1*q2+D*q2*q3
%the following line does the trick
Fnew=subs(F,{q1^2,q2^2,q1*q2,q2*q3},{t1 t2 t12 t23})

[Anis Frej]

I already did that but unfortunately does not work

Just t1 and t2 changes.

[Birdman]

Which version of MATLAB are you using? I am using 2019b and it is working well for me.

[Anis Frej]

I work with matlab R2011a

[Birdman]

See this suggestion:

https://www.mathworks.com/matlabcentral/answers/101283-why-does-the-subs-command-not-substitute-correctly-for-a-symbolic-term-of-variables

[Anis Frej]

Isthere any other method without using the maple function because i have this problem

??? Error using ==> maple at 54

The MAPLE command is not available.

[Birdman]

Is there any chance you can use a newer release of MATLAB?

[Anis Frej]

i will install it

Think you