For if these increases of income or utility are reduced to being infinitesimal, one can use both the symbolism and the powerful manipulations of the differential calculus.
因为，如果这些收入或效用的增加可以化为无穷小，那我们既能使用符号表示，也能利用微分学强大的操作了。
2
Some firms have adopted “differential pricing” schemes that use formulas, based on average income per head, to set lower prices in poor countries.
一些公司已经在采用依据人均收入区别定价的方法向贫穷国家提供价格较低的药品。
3
SRHMS can also discriminate that self rated health is significantly differential in different people, age, family income per person, educational level and profession and residence.