5-7 Adding and Subtracting with Unlike Denominators Course 1 Warm Up Warm Up Lesson Presentation...

5-7 Adding and Subtracting with Unlike Denominators

Course 1

Warm Up

Lesson Presentation

Problem of the Day

Course 1


Warm UpAdd. Write each answer in simplest form.

1. + 2. +

3. + 4. +

17__ 1

7__ 8

15__ 3

15__

79__ 2

9__ 11

20__ 4

20__

27__ 11

15__

134__

Problem of the Day

If it takes 12 minutes to cut a pipe into three pieces, how long would it take to cut the pipe into four pieces.18 minutes

Course 1


Learn to add and subtract fractions with unlike denominators.

Course 1


Vocabulary

least common denominator (LCD)

Insert Lesson Title Here

Course 1


The Pacific Ocean covers of Earth’s surface. The Atlantic

Ocean covers of Earth’s surface. To find the fraction of the

Earth’s surface that is covered by both oceans, you can add

and , which are unlike fractions.

1

3__

1

3__

1

5__

1

5__

Course 1


Fractions that represent the same value are equivalent.

Remember!

Course 1


To add or subtract unlike fractions, first rewrite them as equivalent fractions with a common denominator.

Additional Example 1: Cooking ApplicationMark made a pizza with pepperoni covering of the

pizza and onions covering another . What fraction of

the pizza is covered by pepperoni or onions?

1

4__

1

3__

1

4__

1

4__

1

3__

_____+ Find a common denominator for 4 and 3.

Course 1


?

1

4__ 1

3__

1

4__ 1

3__Add +

Additional Example 1 Continued

Write equivalent fractions with 12 as the common denominator.

1

4__

1

3__

_______+

3

12__

4

12__

_____Add the numerators. Keep the common denominator.

1

4__ 1

3__

7

12__

112

112

112

112

112

112

112

The pepperoni or onions cover of the pizza.7

12__

Course 1


Try This: Example 1Tori made a pizza with peppers covering of the

pizza and ham covering another . What

fraction of the pizza is covered by ham or peppers?

1

3__

1

2__

1

3__ 1

2__

?1

3__

1

2__

_____+ Find a common denominator for 3 and 2.

Course 1


1

3__ 1

2__Add +

Try This: Example 1 Continued

Write equivalent fractions with 6 as the common denominator.

1

3__

1

2__

_______+

2

6__

3

6__

_____Add the numerators. Keep the common denominator.

5

6__

1

3__ 1

2__

16

16

16

16

16

The peppers or ham cover of the pizza.5

6__

Course 1


You can use any common denominator or the least common denominator to add and subtract unlike fractions. The least common denominator (LCD) is the least common multiple of the denominators.

Course 1


Additional Example 2A: Adding and Subtracting Unlike Fractions

Add or Subtract. Write each answer in simplest form.

Method 1: Multiplying denominators.

A. – 7

10__ 3

8__

–7

10__ 3

8__

–56

80__ 30

80__

Multiply the denominators. 10 • 8 = 80

Write equivalent fractions.

Subtract.

Write the answer in simplest form.

26

80__

13

40__

Course 1


Additional Example 2B: Adding and Subtracting Unlike Fractions


Method 2: Use the LCD.

B. – 11

12__ 3

8__

–11

12__ 3

8__

–22

24__ 9

24__

The LCD is 24.


Subtract. 13

24__

Course 1


Additional Example 2C: Adding and Subtracting Unlike Fractions


Method 3: Use mental math.

C. + 5

16__ 1

8__

+5

16__ 1

8__

+5

16__ 2

16__

Think: 16 is a multiple of 8, so the LCD is 16.

Rewrite with a denominator of 16.

Add. 7

16__

1

8__

Course 1


Additional Example 2D: Adding and Subtracting Unlike Fractions



D. – 5

16__ 1

8__

–5

16__ 1

8__

–5

16__ 2

16__



Subtract. 3

16__

Course 1


1

8__

Try This: Example 2AAdd or Subtract. Write each answer in simplest form.

Method 1: Multiplying denominators.

A. – 8

10__ 2

6__

–8

10__ 2

6__

–48

60__ 20

60__

Multiply the denominators. 10 • 6 = 60 Write equivalent fractions.

Subtract.

Write the answer in simplest form.

28

60__

7

15__

Course 1


Try This: Example 2BAdd or Subtract. Write each answer in simplest form.

Method 2: Use the LCD.

B. – 4

7__ 1

3__

–4

7__ 1

3__

–12

21__ 7

21__

The LCD is 21.


Subtract. 5

21__

Course 1


Try This: Example 2C Add or Subtract. Write each answer in simplest form.


C. + 3

12__ 1

6__

+3

12__ 1

6__

+3

12__ 2

12__



Add. 5

12__

1

6__

Course 1


Try This: Example 2D Add or Subtract. Write each answer in simplest form.


D. – 3

12__ 1

6__

–3

12__ 1

6__

–3

12__ 2

12__



Subtract. 1

12__

1

6__

Course 1


Lesson Quiz


1. + 2. –

3. – 4. +

5. Bonnie is making oatmeal bars and the recipes calls for

cup of brown sugar. If she has cup of brown sugar, how

much more does she need?

Insert Lesson Title Here

3

8__ 1

6__ 1

2__ 1

8__

5

12__ 1

6__ 1

6__ 1

8__

3

4__

1

3__

13

24__ 3

8__

1

4__ 7

24__

5

12__ cup

Course 1


5-7 Adding and Subtracting with Unlike Denominators Course 1 Warm Up Warm Up Lesson Presentation...

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