The mathematical equation \lim_{x \rightarrow a} f (x) = \infty states that whenever x is closer to but not equal to a, then the function f(x) is a large positive number. A limit having a value of infinity means that when x approaches to a, the function f(x) becomes bigger and bigger. It means that the function increases without any limit or boundary.

Similarly, the mathematical equation \lim_{x \rightarrow a} f (x) = \infty states that as x approaches to a, the function f(x) is a large positive number, and as x approaches to a, the value of the function f(x) decreases without limit.

Whenever we say that lim = , it means that the limit does not exist. The limit \lim _ {x \rightarrow a} f(x) = L exists only if L is a number. Infinity is not a number as it simply means that something exists without a  boundary. If the limit is positive, then it means that the function increases without any limit. If the limit is negative, then it simply means that the function decreases without any limit.

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Definition of Infinite Limits

In this section, we will define three types of infinite limits.

a) Infinite limits from the left

Suppose f(x) is a function that is defined at all the values in an open interval of the form (b, a)

  • If the values of the function f(x) increase without limit as the values of x (where x is less than a) get closer to the number a, then we can say that the limit as x gets closer to a, from the left is positive infinity and we can write it as:

\lim _ {x \rightarrow a ^{-}} f(x) = + \infty

  • If the values of the function f(x) decrease without limit as the values of x (where x is less than a) get closer to the number a, then we can say that the limit as x gets closer to a, from the left is negative infinity and we can write it as:

\lim _ {x \rightarrow a ^{-}} f(x) = - \infty

 

b) Infinite limits from the right

  • If the values of the function f(x) increase without limit as the values of x (where x is greater than a) get closer to the number a, then we can say that the limit as x gets closer to a, from the left is positive infinity and we can write it as:

\lim _ {x \rightarrow a ^{+}} f(x) = + \infty

  • If the values of the function f(x) decrease without limit as the values of x (where x is greater than a) get closer to the number a, then we can say that the limit as x gets closer to a, from the left is negative infinity and we can write it as:

\lim _ {x \rightarrow a ^{+}} f(x) = - \inftylimxaf(x)=+.(2.4.2)(2.4.2)limx→a−f(x)=+∞.

c) Two sided infinite limits

Suppose the function f(x) is defined for all in an open interval that contains a. Then,

  • If the values of the function f(x) increase without limit as the values of x (where x is not equal to a), get closer to the number a, then we can say that the limit as x becomes closer to the number a is positive infinity. We can mathematically write it as:

\lim _ {x \rightarrow a} f (x) = + \infty

  • If the values of the function f(x) decrease without any limit as the values of x (where x is not equal to a) get closer to the number a, then we can say that the limit as x gets closer to the number a is negative infinity. We can write it mathematically like this:

\lim _ {x \rightarrow a} f (x) = - \infty

It is important to consider a point that when we say that \lim_{x \rightarrow a} f(x) = + \infty or \lim_{x \rightarrow a} f(x) = - \infty, then we are telling the behavior of the function. We are not telling through these notations that the limit exists.

Examples

Evaluate the following limits:

1) \lim _ {x \rightarrow {0 ^{-}}} \frac{1}{2x}

The values of the function decrease without limit as x becomes closer to 0 from the left. Hence, we can conclude that:

\lim _ {x \rightarrow {0 ^{-}}} \frac{1}{2x}= - \infty

 

2) \lim _ {x \rightarrow {0 ^{+}}} \frac{1}{2x}

The values of the function increase without limit as x becomes closer to 0 from the right. Hence, we can conclude that:

\lim _ {x \rightarrow {0 ^{+}}} \frac{1}{2x}= + \infty

 

3) Evaluate \lim_{x \rightarrow 4 ^ {-}} \frac {8} {(x - 4)^3}

We know that if , then we are acknowledging the fact that the value of x is less than 4. Hence, we can say that:

\lim_{x \rightarrow 4 ^ {-}} \frac {8} {(x - 4)^3} = - \infty

 

4) Evaluate \lim_{x \rightarrow 4 ^ {+}} \frac {8} {(x - 4)^3}

We know that if , then we are acknowledging the fact that the value of x is greater than 4. Hence, we can say that:

\lim_{x \rightarrow 4 ^ {+}} \frac {8} {(x - 4)^3} = + \infty

 

 

limxa+f(x)=.(2.4.5)(2.4.5)limx→a+f(x)=−∞
limxaf(x)=+.(2.4.6)(2.4.6)limx→af(x)=+∞.
limxaf(x)=

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Emma

Emma

I am passionate about travelling and currently live and work in Paris. I like to spend my time reading, gardening, running, learning languages and exploring new places.