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## Calculus, all content (2017 edition)

### Unit 1: Lesson 10

Intermediate value theorem

# Intermediate value theorem

Introduction to the Intermediate value theorem. If f is a continuous function over [a,b], then it takes on every value between f(a) and f(b) over that interval.

## Video transcript

- [Voiceover] What we're gonna cover in this video is the intermediate value theorem. Which, despite some of this mathy language you'll see is one of the more intuitive theorems possibly the most intuitive theorem you will come across in a lot of your mathematical career. So first I'll just read it out and then I'll interpret it and hopefully we'll all appreciate that it's pretty obvious. I'm not going to prove it here. But, I think the conceptual underpinning here is it should be straightforward. So the theorem tells us that suppose F is a function continuous at every point of the interval the closed interval, so we're including A and B. So it's continuous at every point of the interval A, B. Let me just draw a couple of examples of what F could look like just based on these first lines. Suppose F is a function continuous at every point of the interval A, B. So let me draw some axes here. So that's my Y axis. And this is my X axis. So, one situation if this is A. And this is B. F is continuous at every point of the interval of the closed interval A and B. So that means it's got to be for sure defined at every point. As well, as to be continuous you have to defined at every point. And the limit of the function that is recorded at that point should be equal to the value of the function of that point. And so the function is definitely going to be defined at F of A. So it's definitely going to have an F of A right over here. That's right over here is F of A. Maybe F of B is higher. Although we can look at different cases. So that would be our F of B. And they tell us it is a continuous function. It is a continuous function. So if you're trying to imagine continuous functions one way to think about it is if we're continuous over an interval we take the value of the function at one point of the interval. And, if it's continuous we need to be able to get to the other, the value of the function at the other point of the interval without picking up our pencil. So, I can do all sorts of things and it still has to be a function. So, I can't do something like that. But, as long as I don't pick up my pencil this is a continuous function. So, there you go. If the somehow the graph I had to pick up my pencil. If I had to do something like this oops, I got to pick up my pencil do something like that, well that's not continuous anymore. If I had to do something like this and oops, pick up my pencil not continuous anymore. If I had to do something like wooo. Whoa, okay, pick up my pencil, go down here, not continuous anymore. So, this is what a continuous function that a function that is continuous over the closed interval A, B looks like. I can draw some other examples, in fact, let me do that. So let me draw one. Maybe where F of B is less than F of A. So it's my Y axis. And this is my X axis. And once again, A and B don't both have to be positive, they can both be negative. One could be, A could be negative. B could be positive. And maybe in this situation. And F of A and F of B it could also be a positive or negative. But let's take a situation where this is F of A. So that, right over there, is F of A. This right over here is F of B. F of B. And once again we're saying F is a continuous function. So I should be able to go from F of A to F of B F of B draw a function without having to pick up my pencil. So it could do something like this. Actually I want to make it go vertical. It could go like this and then go down. And then do something like that. So these are both cases and I could draw an infinite number of cases where F is a function continuous at every point of the interval. The closed interval, from A to B. Now, given that there's two ways to state the conclusion for the intermediate value theorem. You'll see it written in one of these ways or something close to one of these ways. And that's why I included both of these. So one way to say it is, well if this first statement is true then F will take on every value between F of A and F of B over the interval. And you see in both of these cases every interval, sorry, every every value between F of A and F of B. So every value here is being taken on at some point. You can pick some value. You can pick some value, an arbitrary value L, right over here. Oh look. L happened right over there. If you pick L well, L happened right over there. And actually it also happened there and it also happened there. And this second bullet point describes the intermediate value theorem more that way. For any L between the values of F and A and F of B there are exists a number C in the closed interval from A to B for which F of C equals L. So there exists at least one C. So in this case that would be our C. Over here, there's potential there's multiple candidates for C. That could be a candidate for C. That could be a C. So we could say there exists at least one number. At least one number, I'll throw that in there, at least one number C in the interval for which this is true. And, something that might amuse you for a few minutes is try to draw a function where this first statement is true. But somehow the second statement is not true. So, you say, okay, well let's say let's assume that there's an L where there isn't a C in the interval. Let me try and do that. And I'll draw it big so that we can really see how obvious that we have to take on all of the values between F and A and F of B is. So, let me draw a big axis this time. So that's my Y axis. And, that is my X axis. And I'll just do the case where just for simplicity, that is A and that is B. And let's say that this is F of A. So that is F of A. And let's say that this is F of B. Little dotted line. All right. F of B. And we assume that we we have a continuous function here. So the graph, I could draw it from F of A to F of B from this point to this point without picking up my pencil. From this coordinate A comma F of A to this coordinate B comma F of B without picking up my pencil. Well, let's assume that there is some L that we don't take on. Let's say there's some value L right over here. And, and we never take on this value. This continuous function never takes on this value as we go from X equaling A to X equal B. Let's see if I can draw that. Let's see if I can get from here to here without ever essentially crossing this dotted line. Well let's see, I could, wooo, maybe I would a little bit. But gee, how am I gonna get there? Well, without picking up my pencil. Well, well, I really need to cross that line,all right. Well, there you go. I found, we took on the value L and it happened at C which is in that closed interval. So once again, I'm not giving you a proof here. But hopefully you have a good intuition that the intermediate value theorem is kind of common sense. The key is you're dealing with a continuous function. If you make its graph if you were to draw it between the coordinates A comma F of A and B comma F of B and you don't pick up your pencil, which would be true of a continuous function. Well, it's going to take on every value between F of A and F of B.