In economic problems, x 1 and x2 are both commodities, and there are functions (substitution, complementarity, neutrality and aversion) between commodities, and there is a third utility function. Analyze the consumers first, and then tell the producers opposite you. It's basically symmetry. When you understand a problem, add a "negative sign" to your mind.

Solving problems is not afraid of many variables, but they are very similar.