How to Make Complex Symbolic Variable Change?

Hello! Imagine I have a very long equation with many variables, like the example bellow

eq = 0.001*(wr + wl) + 2*z + 1.5*z == -2wp

How can I force matlab to make the following change of variable:

Y = 0.1*(wr + wl) ?

I mean, I need matlab to group the equation (eq) into 0.1*(wr + wl) and replace those terms by Y. In the example above, the result should be:

eq = 0.01*Y + 2*z + 1.5*z == -2wp

Thanks a lot!

Answer 1

[madhan ravi]

>> syms wr wl z wp Y
   eq = 0.001*(wr + wl) + 2*z + 1.5*z == -2*wp
   eq=subs(eq,0.001*(wr+wl),0.01*Y)
 
eq =
 
wl/1000 + wr/1000 + (7*z)/2 == -2*wp
 
 
eq =
 
Y/100 + (7*z)/2 == -2*wp
 
>> vpa(eq)
 
ans =
 
0.01*Y + 3.5*z == -2.0*wp
 
>> 

[Giulio Cesare Mastrocinque Santo]

Hello madhan! Thank you for your answer! I thought about this solution too! The only problem here is that i need to know the term 0.001*(wr + wl) in order to make the exact substitution. However, if my equation is too long, I would need to make the substitution for each individual term. Let's take another equation as an example:

syms wr wl z wp Y x
eq = 0.001*(wr + wl) + sin(2*x)*2.5*(wr + wl)^2 + 0.9*(wr + wl)^2 + 2*z + 1.5*z == -2*wp
subs(eq,0.001*(wr+wl),0.01*Y)

In this case, subs(eq,0.001*(wr+wl),0.01*Y) would only replace the first term. What I wanna knoe is if there is a generic way to force matlab replace Y = 0.1*(wr + wl) in any situation.

[Giulio Cesare Mastrocinque Santo]

Your idead pointed me to the right solution! Thanks a lot! I just need to do the opposit:

syms wr wl z wp Y x
eq = 0.001*(wr + wl) + sin(2*x)*2.5*(wr + wl)^2 + 0.9*(wr + wl)^2 + 2*z + 1.5*z == -2*wp
subs(eq,(wr+wl),Y/0.1)

[madhan ravi]

Anytime :)