Lesson

13 

Comparing Multiplication and Division of Fractions Problem Solving:

Working With a Compass

Comparing Multiplication and Division of Fractions

How do we compare the algorithms for multiplication and division of fractions? We learned to multiply and divide fractions. We also learned to simplify fractions when the answers were not in their simplest form. Let’s review all these procedures.

Multiplication Simplify the answer using the greatest common factor (GCF). 3 2 4·3

We begin by multiplying across.

· ·

3 2 6 4 3 = 12

Next we look to see if the answer is in its simplest form. It’s not. We see that the GCF of 6 and 12 is 6. We pull out 6 from both the numerator and the denominator. 6 6 1 12 = 6 · 2

The simplified answer is 12 .

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Lesson 13

Use the commutative property, then simplify. We have a 3 in the numerator and a 3 in the denominator. This means we can use the commutative property. 3 2 3 4·3=3 =3 3



· 24 ·

·2 4

=1· 2 4



In this case, the fraction still needs to be simplified. We pull out the GCF of 2. 2 2 1 4=2·2 =1· 1 2

The simplified answer is 12 .

Division Divide, then simplify the answer. 3 5 4÷6

We begin by inverting the second fraction. 3 6 4·5

Then we multiply across.

· ·

3 6 18 4 5 = 20

Now we simplify the answer by pulling out the GCF. 2 9     18 20 = 2 · 10 9      = 1 · 10

The simplified answer is 190 .

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Unit 2 • Lesson 13

Lesson 13

When we learn both multiplication and division at the same time, we can sometimes confuse the procedures. However, we can learn certain things that teach us good number sense to avoid making mistakes.

Improve Your Skills It is easy to make a mistake if you forget what operation you are working with. 3 3 5·6

To solve a multiplication problem, you begin by multiplying across. If you forget what operation you are working with, you might invert and multiply.

· ·

3 3 3 6 18 5 · 6 = 5 3 = 15



ERROR

To check that your answer is correct, look at the size of the answer. Remember, when we take a fraction of a fraction, we usually end up

with a smaller number. In terms of fractions, 18 15 is not a small number. 15 3 It is greater than 1 because 15 is 1, and there are 15 left over. This should clue us in that our answer is not correct. When we solve the problem the correct way, the answer looks like this: 3 3 9 5 · 6 = 30



CORRECT

To simplify the answer, we pull out the GCF of 3:

It’s important to think about the size of the fractions and the size of the answer.

9 3 3 3 30 = 3 · 10 = 1 · 10

The simplified answer is 130 . This is much smaller than 18 15 . It is closer to 0 on the number line than 1. Here is where the numbers are located on a number line. 0

3 10

1

18 15

Unit 2 • Lesson 13   151

Lesson 13

We use various tools such as number lines to help us find the error. We can also use common sense about the size of the answer. Let’s look at another example.

Improve Your Skills Here is a division problem. 4 1 5÷5

Suppose you solve it using the algorithm for multiplication, which is to multiply across in the first step.

· ·

4 1 4 1 4 5 ÷ 5 = 5 5 = 25



ERROR

This answer is very small. The answer to a division problem should be 4 bigger. How many of the small units 1 5 are there in 5? Let’s compare it to the actual answer.

· ·

4 1 4 5 4 5 20 5÷5=5·1=5 1= 5



CORRECT

To simplify this answer, we pull out the GCF of 5:



20 5 4 5 =5·1 =1· 4 1



=4



There are 4 units of 15 in the number 4 5.

The answer should be 4, which is quite a bit bigger than 1 5 . We can see this on a number line. 0

1 5

1

Apply Skills Turn to Interactive Text, page 81.

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Unit 2 • Lesson 13

2

3

Reinforce Understanding Use the mBook Study Guide to review lesson concepts.

4

Lesson 13 Problem Solving: Working With a Compass

How do we make a right angle using a compass?

Vocabulary bisect

In the last lesson, we learned how to make a perpendicular line using a compass and a ruler. We made two circles and then drew a perpendicular line to make the right angle. Here is another way to make a perpendicular line and a right angle. Steps for Making a Right Angle Using a Compass Step 1 Start by drawing a line segment. Step 2 Put the sharp end of the compass at one end of the line segment. Stretch the compass so that the pencil end is at the other end of the line segment. Draw arcs above and below the line segment, as shown.

Step 3 Flip over the compass so that the sharp end of the compass is at the other end of the line segment. Now draw two other arcs above and below the line.

Unit 2 • Lesson 13   153

Lesson 13

Step 4 Draw a perpendicular line between the points where the arcs overlap. Now we have a right angle. We can use a protractor to check that our lines are perpendicular.

70 60

50 0

40 0 14

70

12

0 13

60

0

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30

0

30

110

0

15

80

15 20

160

10

170

0

180

0 10 20 180 170 160

90

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Unit 2 • Lesson 13

100

0

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1

100 110

90

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Lesson 13

We can do more than make perpendicular lines with a compass. We can also use a compass and ruler to split an angle into two equal parts. This is called bisecting an angle. It doesn’t matter how big the angle is, we just need a compass and a ruler.

How do we bisect an angle using a compass? Steps for Bisecting an Angle Using a Compass Step 1 Begin by drawing an angle with only a ruler.

Step 2 Put the sharp end of the compass at the vertex of the angle and draw an arc that crosses both sides of the angle.

Unit 2 • Lesson 13   155

Lesson 13

Step 3 Next put the sharp end of the compass at the point where the arc crosses one side of the angle. Make an arc as shown.

Step 4 Last, put the sharp end of the compass on the other point where the arc crosses the side of the angle. Draw another arc. Now we can draw a line from the vertex through the point where the two arcs cross. Our line splits in half, or  bisects , the angle. We can check this by measuring the angles with a protractor.

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Problem-Solving Activity

Reinforce Understanding

Turn to Interactive Text, page 82.

Use the mBook Study Guide to review lesson concepts.

Unit 2 • Lesson 13

Lesson 13

Homework Activity 1 Multiply, divide, and simplify fractions. 1. 4.

12 3 40 10 3 2 12 2 ÷4 4 3 5

· 48

20 1 18 9 6 2 12 4 9 ·3 27 9 5

3

2. 6 ÷ 4

3.

5.

6.

8 63 3 8 30 5 ÷ 10 40 8 9

· 17

3 4

Activity 2 Tell which of the lines are perpendicular. Write the letter on your paper. (a)

(b)

(d) 

(c)

(e) 

Activity 3 Tell which of the triangles have perpendicular lines. Write the letter on your paper. (b) 

(a)

(c) 

(d)

(e)

Activity 4 • Distributed Practice Solve. 1.

3 4

4.

6 8

+ 19 31

36 1 2 − 8 4 2 4

4

3

2. 8 + 6 5.

24 24

3,098 + 1,913

3. 6.

5 9

 

− 16 14

36

7 18

7q574

5,011

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Unit 2 • Lesson 13   157