Learn the limit to tell the infinitesimal times infinity is equal to how much, why?

Learn the limit to tell the infinitesimal times infinity is equal to how much, why?

Any value is OK;
The limit of infinitesimal 1 / N * infinity an is a;
The limit of infinitesimal 1 / N ^ 2 * infinitesimal n is 0;
Infinitesimal 1 / N * infinity the limit of n ^ 2 is infinity