Continuity
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discontinuities, removable vs. nonremovable; zeroes, holes and asymptotes 
external links you may find helpful 
Continuity FAQs
1. If a function has a nonremovable discontinuity at x = c, does x = c have to be a vertical asymptote? 
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Continuity is something best learned from graphs to get a feel for it. Then you can go to equations to cement the concept in your head.
There are 3 parts to continuity. At this moment we are talking about continuity at a point.
For a function f(x) to be continuous at a point x = c all three of these conditions must hold.
1. \( f(c) \) is defined. 
2. \( \displaystyle{\lim_{x \rightarrow c}{f(x)}} \) exists. 
3. \( \displaystyle{\lim_{x \rightarrow c}{f(x)} = f(c)}. \) 
If any one of these conditions is broken, then the function is not continuous at \(x=c\).
Let's look at graphs where each of the above conditions does not hold.
Condition 1: \( f(c) \) is not defined.
This graph shows an example of where the function is not defined at \(x=c\). So this function is not continuous at \(x=c\).
This is an example of a removable discontinuity. We can just redefine the function by redefining one point at \(f(c)\) to make it continuous.
Condition 2: \( \displaystyle{\lim_{x \rightarrow c}{f(x)}} \) does not exist.
Notice that the limit from the left is different than the limit from the right ( at \(x=c\) ). This means the limit does not exist.
This is an example of a nonremovable discontinuity at \(x=c\). There is not way to 'plug the hole' or redefine the function at only one point so that the result is continuous.
Condition 3: \( \displaystyle{\lim_{x \rightarrow c}{f(x)} \neq f(c)}. \)
This graph shows an example of where the first two cases hold but the third doesn't, i.e. \(f(c)\) is defined, the limit exists but the limit does not equal \(f(c)\).
This is also an example of a removable discontinuity. Notice you can just move the \(f(c)\) to fill the hole to make the function continuous.
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Notes
1. Although not explicitly stated above, continuity holds in both directions, i.e. if a function is continuous then all three conditions hold and if all three conditions hold, then the function is continuous. So we can say, \(f(x)\) is continuous at \(x=c\) if and only if all three conditions listed above hold.
2. For case 2 above, where the limit must exist, sometimes we need to look at onesided limits, i.e. limits from each side of the value we are talking about. You will find discussion, videos and practice problems on the onesided limits page for this case.
Example Functions   There are some functions that are guaranteed to be continuous on their domains. This is important . . . these functions are not necessarily continuous everywhere but they are continuous on their domains. We can use this information to build continuity information about other functions.
function type  example 

polynomials  \(x^3+3x^2+1\) 
rational functions  \( (x+3)/(x^21) \) 
root functions  \( \sqrt{x+7} \) 
trig functions  \( \sin(x) \) 
logarithm functions  \( \ln(x1) \) 
Okay, now that you have an intuitive idea of continuity, let's watch some videos to help you understand and use continuity. It is important to watch both of them to get a complete picture of continuity.
1. This video explains continuity from a more mathematical viewpoint.  
Okay, now that you have a better understanding of continuity, take a look at discontinuities explained in the next panel. There is a great video in this section that will help you a lot.
Discontinuities, Removable vs. Nonremovable; Zeroes, Holes and Asymptotes
Discontinuities, Removable vs. Nonremovable; Zeroes, Holes and Asymptotes
This discussion is going to cover several, seemingly diverse, topics. However, they are related in that the resulting equations are similar.
Discontinuities  Removable vs. Nonremovable 

First let's discuss the 3 main types of discontinuities: jumps, holes and asymptotes. Here are three graphs demonstrating each type.
Vertical Asymptote   This graph shows the equation \(\displaystyle{ f(x) = \frac{1}{x1} }\). At \(x=1\) we have a vertical asymptote. This is a nonremovable discontinuity, i.e. we can't redefine that function at \(x=1\) with one value that will make the function continuous there. 

Hole   This plot shows the graph of the equation \(\displaystyle{ g(x)=\frac{x^21}{x1} }\). At \(x=1\) we have a hole. This is a removable discontinuity since if we add the single point \((1,2)\) to the function, the result is a continuous function at \(x=1\). 

Jump   In this third plot, we are graphing
\(\displaystyle{
h(x) = \left\{
\begin{array}{rcl}
x+1 & & x \leq 1 \\
x^2 & & x > 1
\end{array}
\right.
}\)

Zeroes, Holes and Asymptotes 

Now let's look at the equations for each of these. I have included this with the discussion of discontinuities since two of these are discontinuities and the third, zeroes, are related to the other two but are not discontinuities. For this discussion, we are going to look how the equations are similar.
First let's look at zeroes. Zeroes of a function are sometimes called poles (especially in electrical engineering). Basically, they are points where the graph of a function crosses the xaxis, i.e. where \(y=0\). They are not discontinuities but are important points in mathematics and engineering. If you have a function that is a fraction such as \(\displaystyle{ f(x)=\frac{n(x)}{d(x)} }\), zeroes occur at xvalues where the numerator function is zero but the denominator function is NOT zero. For example, look at the second graph above. A zero occurs at \(x = 1\). You can also say that there is a zero at the point \((1,0)\). By definition, the yvalue is zero, so we usually do not write the point \((1,0)\). We usually just say \(x=1\) or at \(1\).
Okay, let's look at holes. If we have the a function in fraction form that looks like \(\displaystyle{ f(x)=\frac{n(x)}{d(x)} }\), holes occur at xvalues where the numerator AND denominator are both zero at the same xvalue. A hole is a discontinuity. Looking at the second graph above, we have a hole at \(x=1\) because the numerator and denominator of g(x) are both zero at \(x=1\).
Finally, vertical asymptotes occur at xvalues where the denominator is zero but the numerator is NOT zero. Asymptotes are discontinuities. The first graph above shows this case. Notice that at \(x=1\), the numerator of f(x) is 1, but the denominator is zero.
Let's sum this up. For a fractional function in the form \(\displaystyle{ f(x)=\frac{n(x)}{d(x)} }\):
 Zeroes occur at xvalues where \(n(x) = 0\) and \(d(x) \neq 0\)
 Holes occur at xvalues where \(n(x) = 0\) and \(d(x) = 0\)
 Vertical Asymptotes occur at xvalues where \(n(x) \neq 0\) and \(d(x) = 0\)
Here is the same information in table form.
\(n(x) \neq 0\)  \(n(x) = 0\)  Discontinuities? 


\(d(x) \neq 0\)  Zero  No 

\(d(x) = 0\)  Vertical Asymptote  Hole  Yes 
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Intermediate Value Theorem
Intermediate Value Theorem
The intermediate value theorem is used to establish that a function passes through a certain yvalue and relies heavily on continuity.
Intermediate Value Theorem 

For a continuous function, \(f(x)\) on an interval \([a,b]\), if \( t \) is between \(f(a)\) and \(f(b)\), 
Notice that the theorem just tells you that the value \(c\) exists but doesn't tell you what it is or how to find it.
To get the idea of this theorem clear in your head, here are some great videos for you to watch. They use graphs to help you understand what the theorem means.
Here is a video that shows, graphically, how the intermediate value theorem works. She uses color in her graph to make it easy to follow.  
Here is a great video that clearly explains the intermediate value theorem more from a mathematical point of view than in the previous video.  