means "whenever x is close to (but not equal to)
a, then f(x) is a large positive number. In other words,
as x gets closer and closer to a, f(x) gets bigger and bigger
without bound. Likewise, the statement limx→af(x)=−∞
means that "whenever x is close to a, f(x) is a large
negative number." Remember that ∞ is not a number!!
The word "infinity" literally means without
end. If a limit is +∞, then the function grows
without end. If the limit is −∞, it shrinks without end.
Vertical Asymptotes
Definition: The line x=a is called a vertical asymptote of a function f if at least one of the following is true:
limx→af(x)=∞;
limx→af(x)=−∞;
limx→a−f(x)=∞;
limx→a−f(x)=−∞;
limx→a+f(x)=∞;
limx→a+f(x)=−∞.
Here are some examples of graphs with a vertical asymptote.
Vertical Asymptotes for Rational Functions
Rational functions often have vertical asymptotes when the denominator goes to zero (and the numerator doesn't), such as f(x)=3x−4 or f(x)=x2−1x2−4.