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 .
Unit 2 • Lesson 13 149
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 .
150 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.
152 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
50
30
0
30
110
0
15
80
15
20
160
10
170
0
180
0
10
20
180 170
160
90
40
Unit 2 • Lesson 13
100
0
13
1
100
110
90
14
154 20
80
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.
156 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