"...เขียนเป็น สมการ optimization..."

Objective Function and the set of constraints are no more than a 'prescription or set of characteristics' for a (set of) solution(s). They need to be 'well-defined and measurable (mathematically)'. But they need not be described in 'equations'. Problems may be much easier to solved when they are in equations. But AI techniques are used because they are not. ;-)