defining symbolic variables in the system

[skyline]

Hello,

My objective function is rather complicated, and I would like to define a variable that is a function of parameters that I would like to estimate when I define a system.

For example, I want to estimate two variables called theta1 and theta2, but since some part of the objective function is written as a function of theta1+theta2+theta1*theta2, I would like to define a variable theta3=theta1+theta2+theta1*theta2 and want to write a part of the objective function as a function of theta3 (to simply expressions).

So what needs to be done?