我来吧，就是说不考虑预算，你只需用边际效用递减规律。你面前放一堆苹果，你随便吃，第一个对你来说效用为10，第二个为9，以此类推，直到你吃的饱的不行了，在吃一个你就会觉得涨了，不想在吃了，我们可以视为边际效用为0，那么总效用是（10+9+。。。）=max,这下，你在吃一个苹果，此时的效用就为负了b<0。max+b<max.

suppose the utility function is u(x), x>=0. if there is no burget constraint, the problem of max:

u(x) can be solved as: u'(x)=0. this means when the marginal utility is zero, the utility on the

consumption of this commodity is maximized.

In practical words, the more you consume, although the marginal utility is decreasing, the total utility can be increased if the marginal utility is positive. the utility can be increased until the marignal utility become zero.