A function is continuous when its graph is a single unbroken curve that you could draw without lifting your pen from the paper.
Here is a continuous function:
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When a function is continuous within its Domain, it
is a continuous function.
So what is
not continuous (also called
discontinuous) ?
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Look out for holes, jumps or vertical asymptotes (where the function heads up/down towards infinity).
Example: 1/(x−1)
At x=1 we have:
1/(x−1) = 1/0 = undefined
So there is a "discontinuity" at x=1
So f(x) = 1/(x−1) over all Real Numbers is
NOT continuous
Let's change the domain to
x>1
So g(x)
IS continuous
In other words g(x) does not include the value
x=1, so it is continuous.
We can define continuous using Limits:
A function f is continuous when, for every value c in its Domain:
f(c) is defined, and lim f(x) = f(c) as x→c
"the limit of f(x) as x approaches c equals f(c)"
The limit says:
"as x gets closer and closer to c
then f(x) gets closer and closer to f(c)"
And we have to check from both directions:
as x approaches c (from left)
then f(x) approaches f(c)
and
as x approaches c (from right) then f(x) approaches f(c)
If we
get different values from left and right (a "jump"), then
the limit does not exist!
And remember this has to be true for every value c in the domain.
1. Removable discontinuity
What Are Holes?
Another way we can get a removable discontinuity is when the function has a hole. A hole is created when the function has the same factor in both the numerator and denominator. This factor can be canceled out but needs to still be considered when evaluating the function, such as when graphing or finding the range. When dealing with a function like this, there will be some point where the function is undefined. Look at this function, for example.
f(x)=(x^2-4x)/(x-4)........factor numerator
f(x)=(x(x-4))/(x-4)
This function has the factor x - 4 in both the numerator and denominator. What happens at the point x = 4? Let's see.
f(x)=(4(4-4))/(4-4)=(4*0)/0 =0/0 =>the function is
undefined
In the graphs below,
there is a hole in the function at x=a.
These holes are called removable discontinuities.
2.
Infinite discontinuity
If the function doesn't approach a particular finite value, the limit does not exist. This is
an infinite discontinuity.
The following two graphs are also examples of
infinite discontinuities at x=a. Notice that in all three cases, both of the one-sided limits are infinite.
3.
Jump discontinuity
The function is approaching different values depending on the direction x is coming from. When this happens, we say the function
has a jump discontinuity at x=a.
The graph of f(x) below shows a function that is discontinuous at x=a.
In this graph, you can easily see that lim(−f(x))=L as x→a and lim(+f(x))=M as x→a.