The first question: 1/x, when x tends from negative direction, it is negative infinity, not negative infinitesimal. Negative infinity is also a case of infinity.

The second question: What you said is right. There are many ways to find the limit, such as: 1, definition 2, equivalent infinitesimal substitution 3, L'H?pital's law will be learned later, and so on. If you are a freshman, you should mainly use equivalent infinitesimal instead. In addition, you will learn two important limits; When 1, x tends to 0, the x power of (1+ 1/x) =e (natural constant) 2, pinching criterion, when x "y" z, if the x limit exists as a and the z limit exists as a, then the y limit must exist and be a, if a sequence.

The third question: Yes, the algebraic sum or product of finite infinity is still infinite. The infinitesimal rule applies to infinity.

Question 4: 0 is a special infinitesimal, and the only constant is infinitesimal, which is a special case of infinitesimal. The definition of infinitesimal is: given a function, when the variation tends to a certain value, the function limit is 0, then the function when the variation tends to a certain value is called infinitesimal. Then given a constant function f(x)=0, the limit of the function is 0 no matter what value X tends to, so 0 is the only constant infinitesimal.

Welcome to answer your questions. . .