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Mathematical operations such as addition and subtraction can be performed with rational expressions, as well as multiplication and division. The procedures for adding and subtracting rational expressions are the same as for adding and subtracting numerical fractions. These procedures will be developed in this lesson.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Challenge

Egyptian Fractions

Ramsha is studying Egyptian history for class.She learned that ancient Egyptians had an interesting way to represent fractions. They used unit fractions, which are fractions of the form to represent all other fractions. The examples of the Egyptian fractions was found in the image of the Eye of Horus. Each part of the eye represents a different fraction.

Eye of Horus and Egyptian fractions
External credits: Benoît Stella
The Egyptians were able to write any fraction as a sum of different unit fractions.
However, the Egyptians did not use a given unit fraction more than once, so they would not have written
a How might the Egyptians have expressed
b Find the sum of the rational expressions for any
c Use the result from Part B to show that each unit fraction with can be written as a sum of two or more different unit fractions.
d Describe a procedure for writing any positive rational number, with and as an Egyptian fraction.
Discussion

Least Common Multiple of Polynomials and How to Find It

Concept

Least Common Multiple

The least common multiple (LCM) of two whole numbers and is the smallest whole number that is a multiple of both and It is denoted as The least common multiple of and is the smallest whole number that is divisible by both and Some examples can be seen in the table below.

Numbers Multiples of Numbers Common Multiples Least Common Multiple
and
and

A special procedure exists for finding the of a pair of numeric expressions. The LCM of two relatively prime numbers is always equal to their product.

Coprimes LCM
and
and
and

Polynomials can also have a least common multiple. The LCM of two or more polynomials is the smallest multiple of both polynomials. In other words, the LCM is the smallest expression that can be evenly divided by each of the given polynomials.

Polynomials Factor LCM Explanation
is the smallest expression that is divisible by both and
is the smallest expression that is divisible by both and

Finding the LCM of polynomials requires identifying the factors with the highest power that appear in each polynomial.

Method

Finding the Least Common Multiple of Polynomials

To find the least common multiple (LCM) of two or more polynomials, the polynomials must be factored completely. The LCM is the product of the factors with the highest power that appear in any of the polynomials. To show an example, the LCM of the following polynomials will be found.
The procedure of finding the LCM of the polynomials involves three steps.
1
Factor Each Polynomial and Write Numerical Factors as Product of Primes
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Each polynomial will be factored. Start by factoring the first polynomial. Note that is used in all of its terms. Therefore, it will be factored out.
Then the numerical factor can be written as a product of and
Now the other polynomial will be factored. Since the factor can be seen in each of its terms, start by factoring it out.
Factor
2
Identify the Factors With the Highest Power
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Both polynomials are now written in factored form. Now, write all the missing related factors to identify the factor with the highest power.

Standard Form Factored Form All Related Factors
Polynomail I
Polynomail II
The highest power of each prime factor can be listed as follows.
3
Multiply the Highest Power of the Factors
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The LCM of the polynomials is the product of the factors listed in the previous step.
As shown, the least common multiple of the given polynomials is another polynomial.
Example

Writing an Expression for the Meeting Time

After reading some interesting facts about the Egyptians, Ramsha dreams of cycling around the Great Pyramid of Giza with her friend Heichi.
Pyramid and a circular path around it
Suppose that the time it takes for each of them to complete a lap are represented by the following polynomials.
If they start at the same place and at the same time and go in the same direction, write an expression for the time when they will meet again at the starting point.

Hint

What is the least common multiple of the given polynomials?

Solution

Imagine that a pair of numbers was given instead of a pair of polynomials. For example, suppose Heichi completes a lap in minutes and Ramsha completes a lap in minutes. Then, they would meet at the starting point after minutes, which is the least common multiple of and
Following this reasoning, the least common multiple of the given pair of polynomials needs to be determined.
To do so, each expression will be factored completely. Start by factoring the first polynomial. Note that all its terms contain a common factor,
Now the other polynomial will be factored. The factor can be seen in each of its terms, so start by factoring it out.
Factor
Both polynomials are now factored and the numerical factors are written as a product of prime factors.
Given Form Factored Form All Related Factors
Heichi
Ramsha
The LCM of these polynomials is the product of the highest power of each factor.
This expression represents the time when Heichi and Ramsha meet again at the starting point.
Discussion

Using the Least Common Denominator to Add or Subtract Rational Expressions

Concept

Least Common Denominator

The least common denominator (LCD) of two fractions is the least common multiple (LCM) of the denominators of the fractions. In other words, the least common denominator is the smallest of all the common denominators. Some examples are provided in the table below.

Fractions Denominators Multiples of Denominators Common Denominators LCM of Denominators (LCD)
and and
and and
and and
The least common denominator is used when adding or subtracting fractions with different denominators.
As with fractions, rational expressions must have a common denominator to be added or subtracted. The least common denominator of two rational expressions is the least common multiple of the denominators of the fractions.
Method

Adding and Subtracting Rational Expressions

When adding and subtracting rational expressions, the same rules apply as when adding and subtracting fractions.

Adding and Subtracting With Like Denominators

If the rational expressions have a common denominator, the numerators can be added or subtracted directly.

Here, and are polynomials and

Adding and Subtracting With Unlike Denominators

If the denominators are different, the expressions have to be manipulated to find a common denominator before they can be added or subtracted. One way of doing this is to multiply both numerator and denominator of one of the rational expressions with the denominator of the other, and vice versa.

Here, and are polynomials, and Another way is to find the least common multiple of the denominators, or the least common denominator. Consider adding the following rational expressions to put theory into practice.
The result can be found in four steps.
1
Find the Least Common Denominator
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Recall that the least common denominator is the least common multiple of the expressions in the denominators.
To find the LCM of the polynomials, they need to be factored.
Denominator Factored Form
The least common denominator is the product of the highest power of each prime factor.
2
Rewrite Each Expression with the LCD
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Now the rational expressions will be multiplied by the appropriate factors to obtain the LCD. To do so, expand the first fraction by and the second fraction by
Expand by
Expand by
3
Add the Numerators
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The expressions in the numerators can now be added because both fractions have the same denominator.
4
Simplify the Resulting Expression
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Check if the resulting rational expression can be simplified or not.
Factor the numerator
Simplify
The result is in simplest form. Note that the excluded values for the sum are and
The restrictions on the domain of a sum or difference of rational expressions consist of the restrictions to the domains of each expression.
Note that the process for subtracting rational expressions is similar.
Example

Acidity of the Mouth

Ramsha is very enthusiastic about learning more about Egypt. She finds an Egyptian pen pal, Izabella. Izabella mentions that her favorite dessert is basbousa.

Basbousa.jpg

The talk of dessert reminds Ramsha of something she learned in chemistry class. The pH, or acidity, of a person's mouth changes after eating a dessert. The pH level of a mouth minutes after eating a dessert is modeled by the following formula.
a Simplify the formula.
b What would the acidity of a person's mouth be after minutes? Round the answer to the two decimal place.

Hint

a Rewrite as a fraction and then expand it to have the same denominator as the other fraction.
b Substitute for in the formula from Part A.

Solution

a It is given that the acidity of a person's mouth minutes after eating a dessert can be determined by the following formula.
To simplify the formula, will be rewritten as a fraction whose denominator is
In order to subtract fractions, they need to have a common denominator. The least common multiple of and is so expand the first fraction by this expression.
Now it is possible to subtract the fractions and simplify the expressions.
This is the simplification of the given formula.
b To find the acidity of a person's mouth minutes after they eat a dessert, substitute for in the simplified formula from Part A.
The pH of the person's mouth minutes after eating a dessert would be about
Example

Finding the Time It Takes for a Pipe to Fill a Pool

In one of her emails, Izabella tells Ramsha about her father's job. He is in charge of pool maintenance work at a local hotel.

Pool and three pipes

Her father gives Isabella the following set of information.

  • Three pipes take hours to fill the pool in the hotel.
  • Pipe II takes times as long to fill the pool as Pipe I.
  • Pipe III takes times as long to fill the pool as Pipe II.
Izabella is asked to find how long it takes to fill the pool when only the fastest pipe is open. Izabella and Ramsha bounce ideas off each other but they are unable to come up with a reasonable answer. Help them to find the answer.

Hint

Let be the time it takes for the first pipe to fill the pool. Then, the fraction represents the part of the pool that the first pipe fills in one hour.

Solution

Let be the time it takes for Pipe I to fill the pool. Since Pipe II takes 2 times as long to fill the pool as Pipe I, the time needed for Pipe II to fill the pool will be times Similarly, the time needed for Pipe III will be times because Pipe III takes times as long to fill the pool as Pipe II.

Pipe Time Needed to Fill the Pool (hours)
Pipe I
Pipe II
Pipe III
All of Them

Now, think about how much of the pool is filled by each pipe, alone or together, in one hour. For example, it takes all three pipes hours to fill the pool. Therefore, they would fill of the pool in one hour. The filling rates of the other pipes can be determined by this same logic.

Pipe Time Needed to Fill the Pool (hours) Filling Rate (per hour)
Pipe I
Pipe II
Pipe III
All of Them
The sum of the filling rates of all pipes is equal to the filling rate when all of the pipes are open.
To add the rational expressions, all the denominators must be factored.
Denominator Factored Form
The least common denominator (LCD) is or Now, expand each fraction such that all the denominators are equal to the LCD.
Expand by
Since the denominators on the both sides are the same, the numerators can be set equal. Therefore, This means that it would take hours for the fastest pipe to fill the pool.
Example

Finding the Total Time of Izabella's Trip

Izabella is planning a trip that involves a kilometer bus ride and a high-speed train ride. The entire trip is kilometers.

External credits: Eric Gaba and NordNordWest

The average speed of the high-speed train is kilometers per hour more than twice the average speed of the bus.

a In terms of the average speed of the bus, write an expression for the amount of time Izabella is on the bus and an expression for the amount of time that she rides the train.
b Write an expression for the total duration of trip by using the expressions found in Part A.