# How do rewrite a variable with the symbolic toolbox?

6 views (last 30 days)
TG on 4 Jun 2019
Commented: madhan ravi on 4 Jun 2019
Hi,
I have a question about the symbolic toolbox. When I use simple relations, I cannot rewrite a variable in another variable. here is an example:
syms a b
a=b+3
solve(a,b)
This leads however to -3, which means that solve assumes that a is 0? I would like an output like b=a-3. If I do this in another way using solve, solve cannot find a solution:
syms a b
a==b+3
solve(a,b)
Does anyone have an idea how I can find b=a-3? It isn't necessary to use solve, but I couldn't think of another way. Thanks in advance!

madhan ravi on 4 Jun 2019
syms a b
eq=a==b+3
isolate(eq,b)
%or
b=solve(eq,b)

#### 1 Comment

TG on 4 Jun 2019
Thanks!

TG on 4 Jun 2019
Do you maybe also know how I can fix this problem?
syms a b c
eq1= a==b+3;
eq2= c==2*b-3;
solve(eq2,eq1,a) % This doesn't work, gives an empty matrix
I would like to get an expression of c == 2*(a-3)-3, so I could then solve a in terms of c for example.

#### 1 Comment

madhan ravi on 4 Jun 2019
S = solve(eq2,eq1);
C=S.c

R2018a

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