很简单的问题:
limg(x)=0说明当自变量x趋向于x0,g(x)是无穷小量,即得1/g(x)是无穷大量;
若limf(x)不等于0,则lim[f(x)/g(x)]必为无穷大量,而不会等于常数A,所以limf(x)必等于0.