I'm trying to build a programming function that returns the value of CET so I need to isolate it.
How can I isolate CET so I get something that can I linearly solve using a programming language? Something like CET = ...
This is the formula: $$\sum_{j=1}^n \frac{FC_j}{(1+CET)^{\frac{(d_j-d_0)}{365}}} - FC_0 = 0.$$
There is no general formula for $CET$. This is a generalized polynomial equation, with the term $\frac{1}{1+CET}$ raised to potentially non-integer powers, none of which powers have any obvious relation to the others. The only thing I can think of is to use a good solver in your programming language (which is...?) along with a good estimate of $CET$.