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SAT
Course: SAT > Unit 6
Lesson 4: Passport to Advanced Math: lessons by skill- Solving quadratic equations | Lesson
- Interpreting nonlinear expressions | Lesson
- Quadratic and exponential word problems | Lesson
- Manipulating quadratic and exponential expressions | Lesson
- Radicals and rational exponents | Lesson
- Radical and rational equations | Lesson
- Operations with rational expressions | Lesson
- Operations with polynomials | Lesson
- Polynomial factors and graphs | Lesson
- Graphing quadratic functions | Lesson
- Graphing exponential functions | Lesson
- Linear and quadratic systems | Lesson
- Structure in expressions | Lesson
- Isolating quantities | Lesson
- Function Notation | Lesson
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Operations with rational expressions | Lesson
What are rational expressions, and how frequently do they appear on the test?
Rational expressions look like fractions that have variables in their denominators (and often numerators too). For example, start fraction, x, squared, divided by, x, plus, 3, end fraction is a rational expression. Just as we can add, subtract, multiply, and divide fractions, we can perform the four operations on rational expressions.
Rational expressions also represent the division of one polynomial expression by another. For example, start fraction, x, squared, divided by, x, plus, 3, end fraction represents the division of x, squared by x, plus, 3, for which we can find a quotient and a remainder.
In this lesson, we'll learn to:
- Simplify rational expressions
- Add, subtract, multiply, and divide rational expressions
- Rewrite rational expressions in the form of quotients and remainders
This lesson builds upon the following skills:
On your official SAT, you'll likely see 0 to 2 questions that test your ability to perform operations with rational expressions.
You can learn anything. Let's do this!
How do I simplify rational expressions?
Intro to rational expression simplification
Just like fractions, but with polynomial factoring
You're probably familiar with simplifying fractions like start fraction, 5, divided by, 10, end fraction; we can factor out a 5 from both the numerator and the denominator and cancel them, leaving us with start fraction, 1, divided by, 2, end fraction.
With rational expressions, we can also cancel out factors that appear in both the numerator and the denominator. These factors can be polynomials!
For example, we can simplify the rational expression start fraction, x, plus, 1, divided by, 2, x, plus, 2, end fraction by factoring out x, plus, 1 from both the numerator and the denominator and canceling them, leaving us with start fraction, 1, divided by, 2, end fraction.
On the SAT, the numerators and denominators of rational expressions can also be quadratic expressions and higher order polynomials, so the ability to factor these expressions fluently is key to your success.
Try it!
How do I multiply and divide rational expressions?
Multiplying & dividing rational expressions: monomials
Multiplying and dividing rational expressions
The same rules for multiplying and dividing fractions apply to multiplying and dividing rational expressions.
When multiplying two rational expressions:
When dividing two expressions, recall that dividing by a fraction is equivalent to multiplying by that fraction's reciprocal:
However, to avoid lengthy polynomial operations, it's recommended that you factor and cancel any cancellable factors before you write out the final expression.
To multiply two rational expressions:
- Factor any factorable polynomial expressions in the numerators and the denominators.
- Cancel any identical factors that appear in both the numerators and the denominators of the expressions.
- Multiply the remaining numerators and multiply the remaining denominators.
Dividing two rational expressions is similar to multiplying; just remember that dividing by an expression is equivalent to multiplying by the reciprocal of the same expression.
Let's look at some examples!
What is the product of start fraction, x, squared, divided by, x, plus, 3, end fraction and start fraction, x, plus, 3, divided by, x, end fraction ?
If f, left parenthesis, x, right parenthesis, equals, start fraction, x, squared, plus, 2, x, plus, 1, divided by, x, plus, 3, end fraction and g, left parenthesis, x, right parenthesis, equals, start fraction, x, divided by, x, squared, plus, 4, x, plus, 3, end fraction, what is start fraction, f, left parenthesis, x, right parenthesis, divided by, g, left parenthesis, x, right parenthesis, end fraction ?
Try it!
How do I add and subtract rational expressions?
Adding rational expressions with different denominators
Adding and subtracting rational expressions
The same rules for adding and subtracting fractions apply to adding and subtracting rational expressions.
When adding or subtracting two rational expressions with unlike denominators:
Remember that you can only add and subtract the numerators of the rational expressions if the expressions have a common denominator! In most cases, the easiest way to find a common denominator is to multiply the two unlike denominators, start color #ca337c, b, end color #ca337c and start color #a75a05, d, end color #a75a05.
To add and subtract two rational expressions:
- Find a common denominator for the two expressions. In most cases, the product of the two denominators would work.
- Rewrite the equivalent form of each rational expression using the common denominator.
- Add or subtract the numerators of the expressions while retaining the common denominator.
- Combine like terms and write the result.
- Factor and/or cancel as needed.
Let's look at some examples!
What is the sum of start fraction, x, squared, divided by, x, plus, 3, end fraction and start fraction, 4, x, plus, 3, divided by, x, plus, 3, end fraction ?
What is the difference start fraction, 2, x, divided by, x, plus, 3, end fraction, minus, start fraction, 3, divided by, x, plus, 1, end fraction ?
Try it!
How do I rewrite a rational expression as a quotient and a remainder?
Dividing polynomials by linear expressions
Polynomial long division
We can represent any rational expression as start fraction, a, left parenthesis, x, right parenthesis, divided by, b, left parenthesis, x, right parenthesis, end fraction, where a and b are polynomial expressions in terms of x.
For example, for a, left parenthesis, x, right parenthesis, equals, x, squared, plus, 2, x, plus, 4 and b, left parenthesis, x, right parenthesis, equals, x, plus, 3, start fraction, a, left parenthesis, x, right parenthesis, divided by, b, left parenthesis, x, right parenthesis, end fraction, equals, start fraction, x, squared, plus, 2, x, plus, 4, divided by, x, plus, 3, end fraction.
When dividing a and b, we can find quotient polynomial q and remainder polynomial r such that:
Where the degree of r is less than the degree of b. Since b is usually a first degree polynomial (a, x, plus, b) on the SAT, r is usually a constant.
When dividing two polynomials using long division, we focus on the highest degree terms in the numerator and the denominator first. For example, for start fraction, x, squared, plus, 2, x, plus, 4, divided by, x, plus, 3, end fraction, the highest degree term in the numerator is x, squared, and the highest degree term in the denominator is x. The first question we ask is "what is x, squared divided by x ?"
start fraction, x, squared, divided by, start color #208170, x, end color #208170, end fraction, equals, start color #7854ab, x, end color #7854ab, so we write start color #7854ab, x, end color #7854ab as the first term of the quotient q, find the product of start color #7854ab, x, end color #7854ab and the divisor start color #208170, x, end color #208170, plus, 3, then subtract the product from a. This eliminates the x, squared term in the dividend.
Next, we do the same to what's left of the dividend, minus, x, plus, 4. We ask "what is minus, x divided by x ?"
start fraction, minus, x, divided by, start color #208170, x, end color #208170, end fraction, equals, start color #7854ab, minus, 1, end color #7854ab, so we write start color #7854ab, minus, 1, end color #7854ab as the second term of the quotient q, find the product of start color #7854ab, minus, 1, end color #7854ab and the divisor start color #208170, x, end color #208170, plus, 3, then subtract the product from minus, x, plus, 4. This eliminates the x term in the dividend.
This leaves us with the constant start color #ca337c, 7, end color #ca337c. Since the degree of start color #ca337c, 7, end color #ca337c is lower than the degree of start color #208170, x, end color #208170, plus, 3, we can stop dividing here and write our quotient and remainder.
- q, left parenthesis, x, right parenthesis, equals, start color #7854ab, x, minus, 1, end color #7854ab
- r, left parenthesis, x, right parenthesis, equals, start color #ca337c, 7, end color #ca337c
Therefore, start fraction, x, squared, plus, 2, x, plus, 4, divided by, x, plus, 3, end fraction, equals, start color #7854ab, x, minus, 1, end color #7854ab, plus, start fraction, start color #ca337c, 7, end color #ca337c, divided by, x, plus, 3, end fraction.
Another strategy to find the quotient and the remainder is to group the numerator, which requires us to split the numerator of a rational expression into a polynomial divisible by the denominator and the remainder.
You don't need to know how to group the numerator, but it may save you time on the test.
To divide polynomial expressions a, left parenthesis, x, right parenthesis and b, left parenthesis, x, right parenthesis using long division:
- Divide the highest degree term of a by the highest degree term of b. This gives you a term of the quotient.
- Multiply the result of Step 1 by b.
- Subtract the result of Step 2 from a. Be careful when subtracting negatives!
- Repeat the divide-multiply-subtract steps using what's left of the dividend until the result is of a lower degree than b.
- The terms calculated in the "divide" steps form the quotient q. The leftover polynomial with a lower degree than b is the remainder r.
- Write the result as q, left parenthesis, x, right parenthesis, plus, start fraction, r, left parenthesis, x, right parenthesis, divided by, b, left parenthesis, x, right parenthesis, end fraction.
Example: For f, left parenthesis, x, right parenthesis, equals, x, squared and g, left parenthesis, x, right parenthesis, equals, x, plus, 3, rewrite start fraction, f, left parenthesis, x, right parenthesis, divided by, g, left parenthesis, x, right parenthesis, end fraction in the form q, left parenthesis, x, right parenthesis, plus, start fraction, r, left parenthesis, x, right parenthesis, divided by, g, left parenthesis, x, right parenthesis, end fraction.
Try it!
Your turn!
Want to join the conversation?
At the end of the first video, Khan forgot to add that x cannot equal to 2 either. Both x=-1 and x=2 will result in dividing the polynomial by 0.
Denominator polynomial: x^2 -x -2
When x= -1, we get (-1)^2 -(-1) -2 = 1+1-2 = 0
When x= 2, we get (2)^2 -(2) -2 = 4-2-2 = 0
Therefore, both x=-1 and x=2 are excluded from the solution since the equation would be undefined.
Am I correct?(3 votes)
Technically, you're very correct and you math doesn't have a problem, but normally we use the restriction if we simplify the expression and it loses some information that we can't conclude from the new expression.
Here, you would have absolutely no idea that x couldn't be set to 1 in (x-5)/(x-2) unless you write it in addition to the expression. This makes it important and new information that we couldn't glean from the simplified expression.
If you also write out that x != 2, you can already see that from the denominator of the simplified equation. This makes it redundant, which is why Sal doesn't write it out.(4 votes)
- if I want to prepare foe ACT, where can I find khan academy ACT like this site?(3 votes)
- Khan Academy doesn't have an ACT prep section like it does for the SAT, but many skills and concepts are the same across both tests so you might get some worth out of using KA SAT prep for the ACT, in addition to something else. I'm not sure of any free online source like Khan Academy for the ACT, but there are online courses, as well as prep books and blogs that give you tips, and you can get practice tests from the ACT website and a couple other sources. Best of luck!(4 votes)
They do not add the restrictions to a lot of the "Try it"s which confuses me. They're a part of the answer right?(3 votes)- Hi! Yes, they are absolutely part of the answer, you're totally right! However, these conditions usually are only shown in specific circunstances, like if the question asks for it, or if we're dealing with functions - f(x), for instance. I think they just didn't put it in for convenience's sake.
For instance, if I were to give you y=x-2, and didn't specify that x ∈ lR (x is a real number), you wouldn't really miss it. Even though, technically, I am supposed to, just like in these cases, they are supposed to give you the conditions for each equation.
But you're absolutely right, and it never hurts, I don't think, to go the extra mile, especially if it helps you understand it better!(2 votes)
- Actually i want bto know can we solve
(x^2)/x+3 using the strategy to find the quotient and the remainder is to group the numerator?(2 votes) - can al khwarizmi teach me math lessons on zoom(2 votes)
- In TRY IT part, in explanation part it's written that:
The greatest exponent in the numerator of the rational expression is 3(6x^3).
what 3 means there before the parantheses(1 vote) - there is a typing error above in one of the questions,where instead of ^,6 is written(1 vote)
- At the first question of your turn they need to precise that x is different from 0(1 vote)