9.4 Teacher Edition
9.4 Teacher Edition
Dynamic Classroom.
Learning Target
Convert between different forms
Laurie’s Notes BigIdeasMath.com
COMMON
STATE STANDARDS
of rational numbers. CORE 8.NS.A.1
Success Criteria
● Explain the meaning of Preparing to Teach
rational numbers. ● Students have written fractions and mixed numbers as decimals. They have
● Write fractions and mixed also written terminating decimals as fractions or mixed numbers. Now they
numbers as decimals. will extend their understanding to writing repeating decimals as fractions or
● Write repeating decimals as mixed numbers.
fractions or mixed numbers. ● MP7 Look for and Make Use of Structure: Mathematically proficient students
discern a pattern or structure. Recognizing the equivalence of equations
written in different forms and using the equations to write and solve a third
equation requires that students be able to manipulate equations.
Warm Up
Cumulative, vocabulary, and Motivate
prerequisite skills practice 1 2 3 10
● Ask students to use a calculator to help write the fractions —, —, —,…,— as
opportunities are available in 11 11 11 11
the Resources by Chapter or decimals. To save time, have students work in groups on different fractions
at BigIdeasMath.com. instead of trying to write decimal equivalents for all of them.
● Record the results on the board.
“What patterns do you observe?” Sample answers: All of the decimals have
two repeating digits, and the sum of those two digits is equal to 9. Also, the
ELL Support
first of the two repeating digits is one less than the numerator of the fraction.
Students may know the word ● Explain that today students will learn how to do the reverse of this process,
rational as meaning “reasonable” writing a repeating decimal as a fraction.
or “sensible.” Explain that when
rational is used in math to describe
a number, it has a very specific
Exploration 1
meaning. A rational number is a ● Allow time for students to complete part (a).
a “What was the result of multiplying each number in the first column by 10?”
number that can be written as —,
b Answers will vary. Students should recognize that the decimal points moved
where a and b are integers and b
one place to the right, but the decimals are still repeating.
does not equal 0.
● For part (b), work through the first row as a class. Help students understand
that when you subtract the first equation from the second, 10x = 3.333…
the repeating decimal subtracts out and you are left with a −(x = 0.333…)
Exploration 1 simple equation to solve. 9x = 3
a. 10x = 6.666. . . ; 10x = 1.111. . . ; ● After completing part (b), have students check their 1
x=—
10x = 2.444. . . answers with a calculator. 3
—
● In part (c), work through x = 0.12 as a class. Begin by multiplying the equation
b. See Additional Answers.
by 10 to get 10x = 1.212121….
4 5 3 931 Ask, “When you subtract x = 1.212121… from 10x = 1.212121…, does the
c. —; —; —; —; The procedure
33 11 11 990 repeating decimal subtract out? Explain.” No, you need to multiply
involves writing and solving an
10x = 1.212121… by 10 again, or multiply x = 0.121212… by 100.
equation that does not have a
repeating decimal; The procedure
● Complete the solution and then allow time for students to finish part (c) with
involves multiplying each side their partners. Have students share their work with the class.
of the original equation by 100 ● Three-Minute Pause: Have students discuss and write their explanations for
instead of 10. part (d). See the Formative Assessment Tip on page T-396 for a description of
Three-Minute Pause.
d. Let x equal the repeating decimal
d. Subtract the equation x = d
from the equation 10nx = 10nd,
where n is the number of
repeating digits. Then solve for x.
T-395
x 10x
x 5 0.333 . . . 10x 5 3.333 . . .
x 5 0.666 . . .
x 5 0.111 . . .
x 5 0.2444 . . .
Calculator
b. For each row of the table, use the two
Math Practice equations and what you know about 1÷3
=0.33333
Look for Structure solving systems of equations to write 33333
Why was it helpful to a third equation that does not involve C ÷ ×
multiply each side of a repeating decimal. Then solve
7 8
the equation x 5 d the equation. What does your 9 о
by 10 in part (a)? solution represent? 4 5 6 +
1 2 3 ()
0 . +Шо
c. Write each repeating decimal below =
as a fraction. How is your procedure
similar to parts (a) and (b)? How is
it different?
x 5 0.—
12 x 5 0.—
45
x 5 0.—
27 x 5 0.9—
40
29 29 3 4 116
— 5 — 5 — 5 1.16
25 25 3 4 100
4
So, 1— 5 1.16.
25
5
b. Write — as a decimal.
33
5 a
Because — cannot be written as —, where a is an integer and b is a
33 b
Calculator power of 10, the decimal form of the number does not terminate.
5 ÷ 33
51515
=0.15151 Use long division
Divide 5 by 33. 0.1515
to divide 5 by 33. 33 )‾
5.0000
÷ ×
C −33
8 9 о
7 1 70
5 6 + − 1 65
4
2 3 () 50
1
+Шо = − 33
0 .
170
The remainder repeats.
− 165
So, — 5 0.—
5 So, you can stop.
15. 5
33
Try It
1. 0.2 2. −0.—2
—
3. 4.375 4. 2.54
T-396
T-397
— 124 25
So, 1.25 = — = 1 —.
99 99
Let x = 0.1—6.
x = 0.1—6 Write the equation.
⋅
10 x = 10 (0.1—
6) ⋅ There is 1 repeating digit, so multiply
each side by 101 5 10.
10x = 1.—6 Simplify.
− (x = 0.1—6) Subtract the original equation.
9x = 1.5 Simplify.
1.5
x=— Solve for x.
9
1.5 15 1
The weight of an object on the moon is about — 5 — 5 — times its
9 90 6
weight on Earth.
18. A fun house mirror distorts the image it reflects. Objects reflected in
the mirror appear 1.—3 times taller. When a five-foot-tall person looks
in the mirror, how tall does he appear?
20. DIG DEEPER A low fuel warning appears when a particular car has
—
0.0146 of a tank of gas remaining. The car holds 18.5 gallons of gas and
can travel 36 miles for each gallon used. How many miles can the car
travel after the low fuel warning appears?
Closure
—
● Quick Write: Explain the steps of writing 2.35 as a mixed number.
Learning Target
Convert between different forms
of rational numbers.
Success Criteria
● Explain the meaning of
rational numbers.
● Write fractions and mixed
numbers as decimals.
● Write repeating decimals as
fractions or mixed numbers.
T-398
T-399
(x + 40)° x°
5
16. PRECISION Your hair is — inch long. Write this length as
16
a decimal.
20. 6.0—
89 21. 0.18—
72 22. 11.—
510
26. MODELING REAL LIFE The density of iodine is about 6.28—1 times
the density of acetone. The density of acetone is about 785 kilograms
per cubic meter. What is the density of iodine? Write your answer as
a repeating decimal.
28. MODELING REAL LIFE You and your friend are making
7
pear tarts for a bake sale. Your recipe uses — times the weight
6
of the diced pears used in your friend’s recipe. Your friend’s
recipe calls for 0.3 pound of diced pears. How many pounds of
pears should you buy to have enough for both recipes?
29. PROBLEM SOLVING The table shows the principal and interest earned
per year for each of three savings accounts with simple annual interest. Which
account has the greatest interest rate? Justify your answer.
NUMBER SENSE Determine whether the numbers are equal. Justify your answer.
34. 0.4—
09 + 0.6— 35. −0.— 36. — − 0.— 37. 0.—
03 − 0.—
5 11
81 63 + — 27 04
99 6
9 10
38. STRUCTURE Write a repeating decimal that is between — and —.
7 7
Justify your answer.
34. 1— or 1.—
1
09
11
35. − — or −0.—
58
58
99
36. 1— or 1.5—
37
60
66
37. − — or −0.—
1
01
99
38. Sample answer: 1.—3;
9 — 10 —
Section Resources — = 1.285714 and — = 1.428571
7 7
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