Main content

## Algebra 1

### Unit 8: Lesson 5

Introduction to the domain and range of a function# Intervals and interval notation

Introducing intervals, which are bounded sets of numbers and are very useful when describing domain and range.

We can use interval notation to show that a value falls between two endpoints. For example, -3≤x≤2, [-3,2], and {x∈ℝ|-3≤x≤2} all mean that x is between -3 and 2 and could be either endpoint.

We can use interval notation to show that a value falls between two endpoints. For example, -3≤x≤2, [-3,2], and {x∈ℝ|-3≤x≤2} all mean that x is between -3 and 2 and could be either endpoint.

## Want to join the conversation?

- Instead of writing x<1 or x>1 can I write x<1 U x>1(19 votes)
- You sure can, as x<1 or "x>1" basically means "x<1 U x>1".

Just to make it clear, U is ( as most people who use sets would know ) union. And the union between, suppose A and B ( where A and B are set) which would be written as A U B would mean values that belong to set A or set B. You can also think of it as values/objects that are part of the whole of set A and set B ( A + B ).

Hope that helped. :} :] :)(22 votes)

- Can you please calrify for me what exactly does "real numbers"mean.(8 votes)
- The real numbers are the set of numbers including rational and irrational numbers. So numbers like 6/7, 0.1, 3000, pi, etc. are included. However, a number like "i" is not included. "i" is a complex number. It is equal to the square root of -1. One way to define real numbers is a number that can be plotted on the number line like the one Sal was using in this video. :)(33 votes)

- 9:30Usage of '(' and ')' after and before the infinity denotes its included. However, how can we handle infinity which is just an idea. So is it better to use '[' and ']' brackets before and after the infinity?(6 votes)
- No... you have the symbols reversed. The square brackets indicate the numbers are in the set.

For example: [5, infinity) is the same as x >= 5.

Hope this helps.(21 votes)

- At3:54, Sal said that open circles indicate an open interval. Could't he just make it a closed interval by making it {-0.99999,3.99999} instead of {-1,4}?(4 votes)
- No, but cause values like -0.9999999 and 3.99999999 are also in the solution set.

The solution set must include all possible solutions. This is why open intervals are used. They indicate that we want to start at -1, but not include it and we want all numbers up to but not including the 4.

Hope this helps.(14 votes)

- At8:17,Shouldn't it be {x<1 and x>1} instead of {x<1 or x>1}?(5 votes)
- No... the video if correct. Using the word "and" means that X would need to be both less than 1
**and**greater than 1 at the same time. This is impossible. The word "or" means that you just need one of the conditions to be true.

Hope this helps.(7 votes)

- What is a real number?(4 votes)
- A real number is really just any number that can be defined in an equation and a graph, whether it's rational or irrational (if you're familiar with those terms, you would be able to see where I'm coming from).

Some examples that are real numbers:

8, 1/45, pi symbol (3.1415926...)

Examples of an imaginary numbers (the opposite of a real number):

8/0, square root of -5 (these numbers would be undefined as they aren't what people would see as practically real. If those people were talking mathematically however, which isn't something people would do at a regular basis, then they wouldn't perceive imaginary numbers as something we can put immediately in an equation or graph).

Hopefully that's straightforward enough for you to understand.(3 votes)

- what does Sal mean when he says the real numbers(3 votes)
- If you look at a number line, every point on the number line is a real number. Real numbers include integers, fractions and decimals.(5 votes)

- What if it's all numbers except 1? How would x does not equal 1 would look like in interval notation?(3 votes)
- It would be: (-infinity, 1) U (1, infinity)(5 votes)

- Is there a name for the brackets that look like wavy lines? What do they signify? Thanks.(2 votes)
- I just call them curly brackets. You may see them used in place of regular parentheses in equations and expressions. The one time they must be used is when a set is in roster form of set notation.

Roster form for set or natural numbers: {1, 2, 3, 4 ...}

Set notation: {x | x ϵ Integers and x > 0}

Hope this helps.(7 votes)

- Instead of saying “or”, couldn’t you just use the union symbol?(3 votes)
- Yes the Union (or) and Intersection (and) symbols could be used in place of words, but not everyone knows these symbols.(4 votes)

## Video transcript

- [Voiceover] What I hope
to do in this video is get familiar with the notion of an interval, and also think about ways
that we can show an interval, or interval notation. Right over here I have a number line. Let's say I wanted to talk
about the interval on the number line that goes from
negative three to two. So I care about this-- Let
me use a different color. Let's say I care about this
interval right over here. I care about all the numbers
from negative three to two. So in order to be more
precise, I have to be clear. Am I including negative three and two, or am I not including
negative three and two, or maybe I'm just including one of them. So if I'm including
negative three and two, then I would fill them in. So this right over here, I'm
filling negative three and two in, which means that
negative three and two are part of this interval. And when you include the endpoints, this is called a closed interval. Closed interval. And I just showed you how I can depict it on a number line, by actually
filling in the endpoints and there's multiple ways to talk about this interval mathematically. I could say that this is all of the... Let's say this number line is showing different values for x. I could say these are all of the x's that are between negative three and two. And notice, I have negative
three is less than or equal to x so that's telling us
that x could be equal to, that x could be equal to negative three. And then we have x is less
than or equal to positive two, so that means that x could
be equal to positive two, so it is a closed interval. Another way that we could
depict this closed interval is we could say, okay, we're
talking about the interval between, and we can use brackets because it's a closed interval, negative three and two, and once again I'm using brackets here,
these brackets tell us that we include, this
bracket on the left says that we include negative three,
and this bracket on the right says that we include
positive two in our interval. Sometimes you might see things written a little bit more math-y. You might see x is a member of
the real numbers such that... And I could put these curly
brackets around like this. These curly brackets say that we're talking about a set of values, and we're saying that the set of all x's that are a member of the real number, so this is just fancy math notation, it's a member of the real numbers. I'm using the Greek letter
epsilon right over here. It's a member of the
real numbers such that. This vertical line here means "such that," negative three is less x is less than-- negative three is less than or equal to x, is less than or equal to two. I could also write it this way. I could write x is a
member of the real numbers such that x is a member,
such that x is a member of this closed set, I'm
including the endpoints here. So these are all
different ways of denoting or depicting the same interval. Let's do some more examples here. So let's-- Let me draw
a number line again. So, a number line. And now let me do-- Let me
just do an open interval. An open interval just so that we clearly can see the difference. Let's say that I want to talk about the values between negative one and four. Let me use a different color. So the values between
negative one and four, but I don't want to include
negative one and four. So this is going to be an open interval. So I'm not going to include four, and I'm not going to include negative one. Notice I have open circles here. Over here had closed circles,
the closed circles told me that I included negative three and two. Now I have open circles here,
so that says that I'm not, it's all the values in
between negative one and four. Negative .999999 is going to be included, but negative one is not
going to be included. And 3.9999999 is going to be included, but four is not going to be included. So how would we-- What would be the notation for this? Well, here we could say
x is going to be a member of the real numbers
such that negative one-- I'm not going to say less than or equal to because x can't be equal to negative one, so negative one is strictly less than x, is strictly less than four. Notice not less than or
equal, because I can't be equal to four, four is not included. So that's one way to say it. Another way I could write it like this. x is a member of the
real numbers such that x is a member of... Now the interval is from
negative one to four but I'm not gonna use these brackets. These brackets say, "Hey,
let me include the endpoint," but I'm not going to include them, so I'm going to put the
parentheses right over here. Parentheses. So this tells us that we're
dealing with an open interval. This right over here,
let me make it clear, this is an open interval. Now you're probably
wondering, okay, in this case both endpoints were included,
it's a closed interval. In this case both endpoints were excluded, it's an open interval. Can you have things that
have one endpoint included and one point excluded, and
the answer is absolutely. Let's see an example of that. I'll get another number line here. Another number line. And let's say that we want to-- Actually, let me do it
the other way around. Let me write it first,
and then I'll graph it. So let's say we're thinking
about all of the x's that are a member of the
real numbers such that let's say negative four is
not included, is less than x, is less than or equal to negative one. So now negative one is included. So we're not going to
include negative four. Negative four is strictly less than, not less than or equal to, so x can't be equal to negative
four, open circle there. But x could be equal to negative one. It has to be less than
or equal to negative one. It could be equal to negative one so I'm going to fill
that in right over there. And it's everything in between. If I want to write it with
this notation I could write x is a member of the
real numbers such that x is a member of the interval, so it's going to go between
negative four and negative one, but we're not including negative four. We have an open circle here so I'm gonna put a
parentheses on that side, but we are including negative one. We are including negative one. So we put a bracket on that side. That right over there
would be the notation. Now there's other things that you could do with interval notation. You could say, well hey,
everything except for some values. Let me give another example. Let's get another example here. Let's say that we wanna talk
about all the real numbers except for one. We want to include all
of the real numbers. All of the real numbers except for one. Except for one, so we're gonna
exclude one right over here, open circle, but it can
be any other real number. So how would we denote this? Well, we could write x is a
member of the real numbers such that x does not equal one. So here I'm saying x can be
a member of the real numbers but x cannot be equal to one. It can be anything else, but
it cannot be equal to one. And there's other ways of
denoting this exact same interval. You could say x is a
member of the real numbers such that x is less than one, or x is greater than one. So you could write it just like that. Or you could do something interesting. This is the one that I would
use, this is the shortest and it makes it very clear. You say hey, everything except for one. But you could even do something
fancy, like you could say x is a member of the real
numbers such that x is a member of the set going from
negative infinity to one, not including one, or x is a
member of the set going from-- or a member of the
interval going from one, not including one, all
the way to positive, all the way to positive infinity. And when we're talking
about negative infinity or positive infinity, you
always put a parentheses. And the view there is you
could never include everything all the way up to infinity. It needs to be at least
open at that endpoint because infinity just
keeps going on and on. So you always want to put
a parentheses if you're talking about infinity
or negative infinity. It's not really an endpoint, it keeps going on and on forever. So you use the notation for open interval, at least at that end, and
notice we're not including, we're not including one
either, so if x is a member of this interval or that interval, it essentially could be
anything other than one. But this would have been
the simplest notation to describe that.