How can I substitute two variables into one (insert relation for parallel resistors)

I'm having a problem to reduce expression like this below:

R11a R11b + R11a R12b + R11b R12a + R12a R12b 
--------------------------------------------- 
 C1a C1b C2a C2b R11a R11b R12a R12b R2a R2b

I want use substitution:

R11a * R12a           R11b * R12b 
----------- =R1a;     ------------ =R1b;
R11a + R12a           R11b + R12b

so that I can get this expression:

                 1 
------------------------------- 
C1a C1b C2a C2b R1a R1b R2a R2b

Is it possible?

-Thank You

Answer 1

[David Sanchez]

I guess you are trying to use symbolic variables. You can try with Matlab's Symbolic Math toolbox -> MuPad. It is a much better tool for symbolic calculus. The kind of operation you are trying to perform is not likely to be possible anyway.