Chapter 3 Fractional Notation & Mixed Numerals
SS 3.1 Least Common Multiples
A least common multiple (LCM) is exactly what its name implies, a number that is the least multiple that 2 or more numbers have in common. There are 3 methods of finding the least common multiple of a number. I will introduce them in order of preference. Your book also introduces these three methods. We are learning to find a least common multiple so that we can create a common denominator in order to build a higher term for adding fraction (all that I just said here is that we will be creating equivalent fractions).
Method 1 (Prime Factorization Method)
Step 1 Factor the numbers using prime factorization
Step 2 Take note of the unique factors in the factorizations
Step 3 Create a LCM by using each unique prime factor the number of times that it appears the most, for any one number (not the total number of times that it appears!)
Example: Find the LCM of 12 & 15
Example: Find the LCM of 3, 5 and 7
Note: If they are all primes then the LCM is their product!
Example: Find the LCM of 5, 25, 50
Note: If the largest is a multiple of the smaller then the largest is the LCM.
Method 2 (Listing Multiples and Finding Smallest)
Step 1 List out the multiples of each number
Step 2 Circle the smallest one that all have in common, this is the LCM
We will use the same examples from above to find the LCM using the second method. I think that you will see that in most cases, the first method, although it may seem confusing at first, is superior!
Example: Find the LCM of 12 & 15
Example: Find the LCM of 3, 5 and 7
Example: Find the LCM of 5, 25, 50
The final method is a new method to me, and one that would be useful given very large numbers, but one which would probably be a waste of time for smaller numbers. This method is really a combination of the first 2 methods.
Method 3 (Division by Common Prime)
Step 1 Find the smallest prime which all numbers are divisible by
Step 2 Divide the numbers by that prime and bring the answers down
Step 3 Find a prime that divides at least 2 numbers if possible, and 1 if not
Step 4 Divide the numbers by that prime and bring the answers down, if it is not
possible to divide evenly, bring the number itself down.
Step 5 Continue the process, until you can divide no more
Step 6 The product of the divisor primes and the numbers remaining (those that
can’t be divided anymore) is the LCM
Example: Find the LCM of 12 & 15
Example: Find the LCM of 5, 25, 50
Example: Find the LCM of 180, 100 & 450
HW p. 143-144 #2-52 even & #53
SS3.2 Addition and Applications
In addition we have two cases to consider. The first case is the easiest, when the denominators are alike, and the second requires using material from section 3.1 to find a least common denominator (LCD) to add fractions with unlike denominators. The LCD is the LCM of the denominators, so we have already learned how to find the LCD, and in chapter 2, we learned how to build a higher term when we learned about the Fundamental Theorem of Fractions. We will now put all these skills to use to add fractions with unlike denominators!
Adding Fractions with Common Denominators
Step 1 Add the numerators
Step 2 Bring along the common denominator
Step 3 Simplify if necessary (reduce or change to mixed number)
Example: 2/5 + 1/5
Example: 3/8 + 1/8
Let’s just practice building higher terms first. We have already done this in section 2.5, when we were asked to find the missing number. That was when we learned the Fundamental Theorem of Fractions. The only difference now is that we must determine the denominator for ourselves, and that is where finding the LCM comes in.
Steps for Building a Higher Term
Step 1 Determine the new denominator by finding LCM
Step 2 Set up an equivalent fraction problem using LCM as your new
denominator
Step 3 Find the new numerator by applying the Fundamental Theorem of
Fractions
Example: Build the higher term for each of the following 1/3 & 2/5
Example: Build the higher term for each of the following 2/5, 3/27 & 17/36
Adding Fractions with Unlike Denominators
Step 1: Find the LCM
Step 2: Build equivalent fractions using LCM
Step 3: Add the new fractions with common denominators.
Step 4: Simplify the fraction if possible
Now let’s practice adding with unlike denominators by putting all the steps together.
Example: 1/3 + 2/5
Example: 1/4 + 2/3
Example: 3/8 + 5/12 + 8/15
Application problems when adding fractions are no different than addition problems involving whole numbers. We are still looking for the key words of sum, total, more than, greater than, etc., which will indicate to us that the problem is an addition problem.
Example: Brian bought ¾ pounds of cashews, 3/8 pounds of almonds and 3/2
pounds of peanuts. How many pounds of nuts did Brian buy
altogether?
Example: A guitarist’s band is booked for Friday and Saturday nights at a local
club. The guitarist is part of a trio on Friday and part of a quintet on
Saturday. Thus the guitarist is paid one-third of one-half the
weekend’s pay for Friday and one-fifth of one-half the weekend’s pay
for Saturday. What fractional part of the band’s pay did the guitarist
receive for the weekend’s work? If the band was paid $1200, how
much did the guitarist receive?
Notice that some problems may now contain both addition and multiplication!
HW p. 149-150 #2-52 even & #41
SS 3.3 Subtraction, Order and Applications
Subtraction is the same as addition in method. There is no need to go into a detailed explanation, as the steps are the same. Let’s review the steps and then do some practice problems.
Subtracting Fractions with Like/Unlike Denominators
Step 1: Find the LCM
Step 2: Build equivalent fractions using LCM (if like denominators skip to next
step)
Step 3: Subtract the new fractions with common denominators (watch order)
Step 4: Simplify the fraction if possible
Now let’s practice a few of each type.
Example: 6/8 - 1/8
Example: 5/6 - 1/6
Example: ¾ - 1/8
Example: ¾ - 1/20
Comparing Fractions Using Inequality Symbols
When denominators are alike there is little to comparing fractions, we simply compare the numerators and the larger numerator is the larger fraction. You can think of this like eating a pie. If you have a calorie-free pie cut into 5 equal pieces and someone tells you that you will either get 1 slice (1/5) or 2 slices (2/5) -- Which would you rather have? Well, obviously 2 slices, because that is more pie!! Think of your fraction problems like this pie.
Example: Use < or > to compare the following fractions
a) ¼ ¾
b) 5/8 7/8
c) 121/131 7/131
If the denominators are not alike, then we must go about the comparison in a different manner. Of course, as the book suggests, we could find the LCD and build the higher terms and then make the comparison based upon the numerators, but there is an easier way. We can simply compare the cross products. The fraction whose cross product is larger, is the larger fraction! Recall the discussion from section 2.5.
If I want to tell which of 2 fractions is larger, I cross multiply
5 1 1 4
4 5
The fraction with the larger cross product is the larger fraction. If the cross product is the same then the fractions are equivalent!
Example: Use < or > to compare the following fractions
a) ¾ 8/12
b) 5/6 7/9
c) 2/5 5/8
Hmm, let’s compare my method and the book’s method. Which do you like? A simple cross multiplication or finding the LCD, building the higher term and then fiiiiinally comparing the numerators!
Example: Use < or > to compare the following fractions
2/12 3/28
Book’s Method
My Method
Word problems involving fractions and subtraction are no different than word problems involving whole numbers. When we run into a set up where an addend is missing, we see that we have a subtraction problem, and set the equation up appropriately. Remember that the words difference, less than, how much more, etc. indicate subtraction.
Example: Melaine spent ¾ hours listening to rock and jazz music. She spent 1/3
of an hour listening to jazz. How many hours were spent listening to
rock? How many minutes is this?
Example: As part of a rehabilitation program, an athlete must swim and walk a
total of 9/10 km each day. If one lap in the swimming pool is 3/80 km,
how far must the athlete walk after swimming 10 laps, in order to
complete the 9/10 km requirement?
HW
p. 155-156 #2-50 & 56-68 evens & #74-77 all
SS 3.4 Mixed Numerals
Mixed numbers (as most people call them) are an addition problem with the addition sign left out. Mixed numbers represent the sum of whole and a fractional part. Let’s take a look at what I am talking about by looking at a pictorial representation of a mixed number and how it relates to an improper fraction and addition.
Write as an addition problem.
2 + 1 = 3 Improper Fraction
+ 2 2 2 ¿
How many wholes and how many parts are there in the picture above?
There is whole and parts which equals the mixed number
¿ + ¿ = ¿
As can be seen from the above examples, the mixed number 1 ½ and the improper fraction 3/2 are equivalent, but different ways to write the same number. We need a way to change a mixed number to an improper fraction and vice versa without drawing pictures, however. We will start by changing improper fractions to mixed numbers, since we have already encountered improper fractions.
Improper Fraction ® Mixed Number
Step 1 Divide numerator by denominator and write in remainder form.
Step 2 Use remainder as the numerator of the fractional part. Its denominator is the
denominator of the original improper fraction. Write as a mixed number.
Step 3 (Note that this step can be avoided, if the improper fraction is reduced to begin with) Reduce
the fractional portion
Step 4 Rewrite
Example: Change 3/2 to a mixed number
(Note: Don’t forget the whole number when rewriting!)
Example: Change 7/2 to a mixed number
Example: Change 14/4 to a mixed number
Example: Change 26/4 to a mixed number
(Note: You can recognize that the last 2 examples are not reduced prior to converting them to mixed numbers and reduce before conversion, but the entire method must be demonstrated!)
From this point on, all improper fractions should be converted to mixed numbers. Any answer with an improper fraction must be converted unless specified!
Mixed Numbers ® Improper Fractions
Step 1 Make sure that the fractional portion of the mixed number is in lowest terms
Step 2 Multiply the denominator by the whole number and add the numerator
Step 3 Put the new number from step 2 over the original fractional portion’s denominator
(Note: We will be using this to add, subtract, multiply and divide mixed numbers)
Example: Change 2 ¾ to an improper fraction
Example: Change 7 ½ to an improper fraction
Example: Change 2 1/8 to an improper fraction
The concept of a mixed number comes in quite handy when dealing with a long division problem where there is a remainder involved. Rather than writing the answer as a whole with a remainder, we can now write the answer as a mixed number. The following describes how this is accomplished.
Long Division Answers as Mixed Numbers
Step 1 Divide as normal until a remainder is reached
Step 2 Instead of writing remainder such and such, place the remainder over the divisor
and write next to the whole number portion of the answer.
Step 3 Reduce the fractional portion if necessary
Example: Divide 25é3251
Example: Divide 18 ¸ 8
(Note: This problem requires the fractional portion to be reduced, and that is the reason why I provide the steps that I do for converting an improper fraction to a mixed number. I believe in doing this, you will be more prepared for having to reduce the fractional parts of a long division answer where appropriate.)
Even though our author does not have any problems like these in this section, let’s just throw a couple out there to show the importance of converting an improper fraction to a mixed number.
Example: ½ ¸ 1/3
Example: 2/3 ¸ 1/5
HW
p. 161-162 #2-62 all evens & #64, 70
SS 3.5 Addition & Subtraction with Mixed Numerals & Applications
Although your author does not discuss the fact, there are 2 methods for adding mixed numbers. The first method that I will show is the one shown by the author and the one that I want you to use to complete all homework problems. I will also be specifying some problems to be completed using the second method as well. It is very useful to know and be proficient at both methods. At times one method may be superior to the other, so it is important to learn both!
Adding Whole Numbers & Fractions
Step 1 Place in columns with whole numbers over whole numbers and fractions over fractions, if not already positioned this way
Step 2 If there is not a common denominator, find the LCM and build higher term to the
right of the fractions
Step 3 Add fractions
Step 4 Add whole numbers
Step 5 If fractional part is improper or needs to be reduced do the appropriate thing.
Step 6 Add the whole number and fractional portion together to make a mixed number
Example: 2 9/50 + 3 7/25
Example: 9 4/5 + 11 ½
Example: 17 2/3 + 103 11/27
Example: 283 + 5 3/10
Example: 5 ¾ + 6 ½