I have two variable whose product should remain constant, say A and B are the variables and A*B = C (C=Const.). I want and can change both of the variables in an optimization analysis. However, when I want to introduce this constraint (AB=C) I don't seem to be able to find a way of doing it. If I write either A or B with the use of the constant C, as in A = C/B or B =  C/A, I won't be able to select the variable in optimization tab of the design properties. I also won't be able to set up A*B as a cost function that should be equal to C because I am using minimize as the condition of my main cost function which does not allow the addition of other cost functions. Any thoughts on how to do this?
This issue exists in all the available analyses of the optometrists so one solution should fit all.

Your help is much appreciated.