Infinite Limits
The statement lim tells us that whenever x is close to (but not
equal to) a, f(x) is a large positive number. A limit with
a value of \infty means that as x gets closer and closer to a,
f(x) gets bigger and bigger; it increases
without bound. Likewise, the statement \lim_{x \to a} f(x) = -\infty tells us
that whenever x is close to a, f(x) is a large negative
number, and as x gets closer and closer to a, the value of
f(x) decreases without bound.
Warning: when we say a
limit =\infty, technically the limit doesn't exist.
\displaystyle\lim_{x\to a}f(x)=L makes sense (technically) only
if L is a number. \infty is not a number! (The
word "infinity" literally means without
end.) If the limit is + \infty, then the
function increases without end. If the limit is -\infty, it
decreases without end. We say a limit is equal to
\pm\infty just to indicate this increase or decrease, which is
more information than we would get if we simply said the limit
doesn't exist.
Vertical Asymptotes
Definition: The line x=a is a vertical
asymptote of a function f if the limit of
f as x\to a from the left and/or right is
\pm\infty. This means at least one of the
following is true:
\lim_{x \rightarrow a^+}f(x) =
\infty, |
\lim_{x \rightarrow a^-}f(x) =
\infty, |
\lim_{x \rightarrow a^+}f(x) =
-\infty, |
\lim_{x \rightarrow a^-}f(x) =
-\infty. |
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Here are some examples of graphs with one or more vertical
asymptotes.
DO: Find all vertical asymptotes in the
following graphs.
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