4 ## INFINITY (∞)The definition of "becomes infinite" A LIMIT is a number. Infinity, along with its symbol ∞, is not a number and it is not a place. To become "infinite" is the idiom we use to describe a
DEFINITION 4. "becomes infinite." We say that a variable "becomes infinite" if, beginning with a certain term of a sequence of its values, the absolute value of that term and any subsequent term we might name is greater than any positive number we name, however large.
When the variable is If In both cases, we mean: No matter what large number M we might name, we get to a point in the sequence of values of When the variable is Although we write the symbol "lim" for limit, those algebraic statements mean: The limit of Definition 4 is the definition of "becomes infinite;" it is not the definition of a limit. Thus we employ the symbol ∞ in algebraic statements to signify that the definition of becomes infinite has been satisfied. That symbol by itself has no meaning.
Let us see what happens to the values of As the sequence of values of
negative numbers. In that case, we write When a function becomes infinite as
vertical asymptote. Next, let us consider the case when
We should read that as "the limit as See First Principles of Euclid's Elements, Commentary on the Definitions. See especially that a definition is Finally, when
write In other words, whenever
called a horizontal asymptote of the graph.
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we might name. (Definition 4.) Limits of rational functions A rational function is a quotient of polynomials (Topic 6 of Precalculus). It will have this form:
where Apart from the constant term, each term of a polynomial will have a factor
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According to 1), above, the limit of each term that contains In similar cases, the first step is:
The result follows on dividing both numerator and denominator by
In other words: Problem 4.
In the following, the rational function is the reciprocal of the one above:
This problem illustrates:
Change of variable Consider this limit: Rather than have the variable approach 0, we sometimes prefer that it become infinite. In that case, we do a change of variable. We put
Where will this come up? In the limit from which we calculate the number e : (Lesson 15.)
Problem 5. In the above limit, change the variable to
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