If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Algebra (all content)

Course: Algebra (all content) > Unit 13

Lesson 1: Intro to rational expressions
Math
Algebra (all content)
Rational expressions, equations, & functions
Intro to rational expressions
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

Intro to rational expressions

Google Classroom
Learn what rational expressions are and about the values for which they are undefined.

What you will learn in this lesson

This lesson will introduce you to rational expressions. You will learn how to determine when a rational expression is undefined and how to find its domain.

What is a rational expression?

A polynomial is an expression that consists of a sum of terms containing integer powers of x, like 3, x, squared, minus, 6, x, minus, 1.
A rational expression is simply a quotient of two polynomials. Or in other words, it is a fraction whose numerator and denominator are polynomials.
These are examples of rational expressions:
  • start fraction, 1, divided by, x, end fraction
  • start fraction, x, plus, 5, divided by, x, squared, minus, 4, x, plus, 4, end fraction
  • start fraction, x, left parenthesis, x, plus, 1, right parenthesis, left parenthesis, 2, x, minus, 3, right parenthesis, divided by, x, minus, 6, end fraction
Notice that the numerator can be a constant and that the polynomials can be of varying degrees and in multiple forms.

Rational expressions and undefined values

Consider the rational expression start fraction, 2, x, plus, 3, divided by, x, minus, 2, end fraction.
We can determine the value of this expression for particular x-values. For example, let's evaluate the expression at start color #11accd, x, end color #11accd, equals, start color #11accd, 1, end color #11accd.
2(1)+31−2=5−1=−5\begin{aligned} \dfrac{2(\blueD{1})+3}{\blueD1-2} &= \dfrac{5}{-1} \\\\ &=\goldD{-5} \end{aligned}
From this, we see that the value of the expression at start color #11accd, x, end color #11accd, equals, start color #11accd, 1, end color #11accd is start color #e07d10, minus, 5, end color #e07d10.
Now let's find the value of the expression at start color #11accd, x, end color #11accd, equals, start color #11accd, 2, end color #11accd.
2(2)+32−2=70=undefined!\begin{aligned} \dfrac{2(\blueD{2})+3}{\blueD2-2} &= \dfrac{7}{0} \\\\ &=\goldD{\text{undefined!}} \end{aligned}
An input of 2 makes the denominator 0. Since division by 0 is undefined, start color #11accd, x, end color #11accd, equals, start color #11accd, 2, end color #11accd is not a possible input for this expression!

Domain of rational expressions

The domain of any expression is the set of all possible input values.
In the case of rational expressions, we can input any value except for those that make the denominator equal to 0 (since division by 0 is undefined).
In other words, the domain of a rational expression includes all real numbers except for those that make its denominator zero.

Example: Finding the domain of start fraction, x, plus, 1, divided by, left parenthesis, x, minus, 3, right parenthesis, left parenthesis, x, plus, 4, right parenthesis, end fraction

Let's find the zeros of the denominator and then restrict these values:
(x−3)(x+4)=0↙↘x−3=0orx+4=0x=3orx=−4\begin{aligned} (x-3)&(x+4)= 0 \\\\ \swarrow \quad&\quad \searrow \\\\ x-3=0 \quad &\text{or} \quad x+4=0 \\\\ x = 3 \quad&\text{or} \quad x=-4 \end{aligned}
So we write that the domain is all real numbers except start text, 3, end text and start text, negative, 4, end text, or simply x, does not equal, 3, comma, minus, 4.

Check your understanding

Problem 1
What is the domain of start fraction, x, plus, 1, divided by, x, minus, 7, end fraction?
Choose 1 answer:

Problem 2
What is the domain of start fraction, 3, x, minus, 7, divided by, 2, x, plus, 1, end fraction?
Choose 1 answer:

Problem 3
What is the domain of start fraction, 2, x, minus, 3, divided by, x, left parenthesis, x, plus, 1, right parenthesis, end fraction?
Choose 1 answer:

Problem 4
What is the domain of start fraction, x, minus, 3, divided by, x, squared, minus, 2, x, minus, 8, end fraction?
Choose 1 answer:

Problem 5
What is the domain of start fraction, x, plus, 2, divided by, x, squared, plus, 4, end fraction?
Choose 1 answer:

Want to join the conversation?

Log in
  • duskpin sapling style avatar for user Jenn Vo
    Posted 7 years ago. Direct link to Jenn Vo's post “I have a question about #...”
    I have a question about #5 under the Check your understanding section. So for the denominator in that fraction, can I use the method "the different of 2 squares" to factor it out to (x+2) (x-2) and solve for x from there? Can you explain more about it? I didn't get the last part in the explanation. Thanks!
    (22 votes)
    Default Khan Academy avatar avatar for user
    • primosaur ultimate style avatar for user Mr. Brownridge
      Posted 7 years ago. Direct link to Mr. Brownridge's post “Difference refers to subt...”
      Good Answer
      Difference refers to subtraction. x^2+4 is a sum. Therefore, it is a "sum of two squares." If you graph the function you will see that it is an upward facing parabola with a y-intercept of 4. It has no solutions. So the expression will never equal zero (unless we use a different set of numbers called complex numbers).
      (21 votes)
  • blobby green style avatar for user MatthewS
    Posted 3 years ago. Direct link to MatthewS's post “I don't have a good under...”
    I don't have a good understanding of how exactly you find the domain, and what "all real numbers" means.
    (6 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user Victor
      Posted 3 years ago. Direct link to Victor's post “Domain means that you are...”
      Domain means that you are trying to find all possible values of x. Domain's are usually written in this format: {xeR} where xeR means that for every real number, x is a solution. All real numbers mean any number that exists, and they may be irrational, rational, negative, positive, etc. However, they cannot be undefinable values such as √-1, which is i in short. In order to find the domain, you'll have to find what can't be in the denominator usually by factoring, and you'll be able to find out what x cannot be. If you have a specific question you'd like me to walk you through, don't hesitate to ask!
      (9 votes)
  • duskpin ultimate style avatar for user Jedidiah Poh
    Posted 2 months ago. Direct link to Jedidiah Poh's post “this is so fun”
    this is so fun
    (8 votes)
    Default Khan Academy avatar avatar for user
  • duskpin sapling style avatar for user aaliyahmariecerveny
    Posted 4 years ago. Direct link to aaliyahmariecerveny's post “Why is number 5, _all rea...”
    Why is number 5, all real numbers shouldn't it be +/- 2 since x^2=+4, factors out to (x+2)(x-2)?
    (7 votes)
    Default Khan Academy avatar avatar for user
    • stelly blue style avatar for user Kim Seidel
      Posted 4 years ago. Direct link to Kim Seidel's post “The denominator is: x^2+4...”
      The denominator is: x^2+4. You changed it into x^2-4.
      x^2+4 is not factorable. Any real number squared will create a positive value. That positive value plus 4 creates an even larger positive value. There is no value that you can use for X that would cause the denominator to become 0. This is why the answer is that the domain = all real numbers.

      Hope this helps.
      (1 vote)
  • blobby green style avatar for user Dolfo Aaron
    Posted 2 months ago. Direct link to Dolfo Aaron's post “Help me please my brain i...”
    Help me please my brain is hurting :(
    (6 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Mrs. Head
    Posted 3 years ago. Direct link to Mrs. Head's post “Why do you use the term "...”
    Why do you use the term "cancel"? I know a lot of teachers use it and that was what my teachers called it when I was in school. But is this really a mathematically correct term?
    I spend a great deal of time correcting students who just want to "cancel" terms just because they are alike, without understanding that in order for terms to be removed from an expression you have to use a mathematical operation, division or subtraction. Therefore terms can only be "divided out" or "subtracted out". Students will often times cross out or as you say "cancel out" terms that are both in numerators when multiplying terms or both in the denominators. To help resolve this issue my students are only allowed to use correct mathematical operations when simplifying expressions (divide out or subtract out).
    (4 votes)
    Default Khan Academy avatar avatar for user
  • leaf orange style avatar for user hwang
    Posted 3 years ago. Direct link to hwang's post “In rational expression wh...”
    In rational expression why is domain all real number?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • female robot grace style avatar for user loumast17
      Posted 3 years ago. Direct link to loumast17's post “rational expressions depe...”
      rational expressions depend on the denominator for domain. If you know how to find vertical asymptotes and holes, those are what would limit the domain of a rational function. The only time a rational function has a domain of all reals is if the denominator is just 1.

      EDIT

      Thanks to Hecretary Bird for his correction. denominator just has to be a constant, other than 0 still though.
      (2 votes)
  • blobby green style avatar for user Fred Haynes
    Posted 4 years ago. Direct link to Fred Haynes's post “In problem # 3, the denom...”
    In problem # 3, the denominator is x(x+1). Why can't the zero simply be -1? Because -1+1 =0 and x*0=0.

    Thanks in advance.
    (3 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user Yu Aoi
    Posted 2 years ago. Direct link to Yu Aoi's post “if a constant is a polyno...”
    if a constant is a polynomial, is 1/2 a rational expression?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Annalishia Johnson
    Posted 3 years ago. Direct link to Annalishia Johnson's post “is it bad that Im just st...”
    is it bad that Im just starting to understand this subject
    (2 votes)
    Default Khan Academy avatar avatar for user