One-Sided Limits
How to find the one-sided limits (left-hand limit, right-hand limit): definition, 2 examples, and their solutions.
Left-Hand Limit
The left-hand limit of f(x) means
the limit of f(x) as x goes to a
[from the left side].
So the left-hand limit
has a minus sign behind a:
a-.
a- means
the number a little bit smaller than a.
(almost the same)
Right-Hand Limit
The right-hand limit of f(x) means
the limit of f(x) as x goes to a
[from the right side].
So the right-hand limit
has a plus sign behind a:
a+.
a+ means
the number a little bit bigger than a.
(almost the same)
When Does a Limit Exist?
The limit of f(x) exist
when
the left-hand limit and the right-hand limit
are equal.
In other words,
if the left-hand limit and the right-hand limit
are not equal,
then the limit of f(x) does not exist.
Example
First find the left-hand limit.
This is when x → 2-:
x < 2.
So the left-hand limit of f(x) is
the limit of (-x + 6) as x → 2-.
Put 2- into (-x + 6).
Limit of a Function
2- means
the number a little bit smaller than 2.
(almost the same)
So -(2-) = -2.
-2 + 6 = 4
So the left-hand limit is 4.
Next, find the right-hand limit.
This is when x → 2+:
x > 2.
So the right-hand limit of f(x) is
the limit of x2 as x → 2+.
Put 2+ into x2.
2+ means
the number a little bit bigger than 2.
(almost the same)
So (2+)2 = 22.
22 = 4
So the right-hand limit is 4.
The left-hand limit is 4.
The right-hand limit is 4.
So the left-hand limit and the right-hand limit
are equal: 4.
So the limit of f(x) exist: 4.
So 4 is the answer.
Example
First find the left-hand limit.
This is when x → 0-.
So the left-hand limit of f(x) is
the limit of x(x + 1)/|x| as x → 0-.
This is when x → 0-:
x < 0.
So change |x| to -x.
Absolute Value
Cancel the x factors.
Then x(x + 1)/(-x) = -(x + 1).
Put 0- into -(x + 1).
0- means
the number a little bit smaller than 0.
(almost the same)
So 0- = 0.
-(0 + 1) = -1
So the left-hand limit is -1.
Next, find the right-hand limit.
This is when x → 0+.
So the right-hand limit of f(x) is
the limit of x(x + 1)/|x| as x → 0+.
This is when x → 0+:
x > 0.
So change |x| to x.
Absolute Value
Cancel the x factors.
Then x(x + 1)/x = x + 1.
Put 0+ into x + 1.
0+ means
the number a little bit bigger than 0.
(almost the same)
So 0+ = 0.
0 + 1 = 1
So the right-hand limit is 1.
The left-hand limit is -1.
The right-hand limit is 1.
So the left-hand limit and the right-hand limit
are not equal.
Then the limit of f(x) does not exist.
So the limit of f(x) does not exist.