Add or Subtract Fractions with Unlike Denominators

1 / 40

# Add or Subtract Fractions with Unlike Denominators - PowerPoint PPT Presentation

Add or Subtract Fractions with Unlike Denominators. Review: Common Multiple. A number that is a multiple of two or more numbers. Common Multiples of 3 & 6: 3: 3, 6, 9, 12, 15, 18, 21, 24… 6: 6, 12, 18, 24, 30, 36, 42…. Review: Least Common Multiple.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'Add or Subtract Fractions with Unlike Denominators' - ashby

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### Add or Subtract Fractions with Unlike Denominators

Review: Common Multiple
• A number that is a multiple of two or more numbers.

Common Multiples of 3 & 6:

3: 3, 6, 9, 12, 15, 18, 21, 24…

6: 6, 12, 18, 24, 30, 36, 42…

Review: Least Common Multiple
• The smallest common multiple of a set of two or more numbers.

5 = 5, 10, 15, 20, 25, 30

6 = 6, 12, 18, 24, 30, 36

• Step 1: Find the multiples of each denominator.

1

5

= 5, 10, 15, 20, 25, 30

1

+

10

= 10, 20, 30, 40, 50

• Step 2: Determine the LCD. (Remember the LCD = LCM)

1

5 = 5, 10, 15 ,20 ,25 ,30

1

+

10 = 10, 20, 30, 40, 50

• Step 3: Make equivalent fractions using the LCD as the new denominator.

1

=

5

10

1

=

+

10

10

You know that 1/10

is equal to 1/10 so

Put a 1 over the

Bottom 10.

1

=

5

10

1

1

=

+

10

10

To find the top

what do you multiply

the 5 by to get 10.

1

5

10

1

1

+

10

10

That’s right 2. And we know that if we multiply by 2 at the bottom then we must also multiply by 2 at the top.

1

2

x 2 =

5

10

x 2 =

1

1

+

10

10

1

2

=

• Step 4: Add/subtract the numerators and keep the denominators the same.

5

10

Remember when

adding fractions the denominators always stay the same!!!!!

1

1

=

+

+

10

10

3

10

• Step 5: Check to make sure your answer is in simplest form.

3:

1 x 3

10:

1 x 10

2 x 5

3

10

Common Factors: 1

3/10 is in simplest form

Find the

common

Multiples for

5 and 3. Write

This number

denominator.

2

5

15

1

+

3

15

5 = 5, 10, 15, 20, 25, 30

3 = 3, 6, 9, 12, 15

what you

multiply the

bottom number

by to get 15.

2

5

15

x 3 =

1

+

3

x 5 =

15

Multiply the

top number

by the same

number you

did in the

bottom.

2

6

x 3 =

5

15

x 3 =

1

5

x 5 =

+

3

x 5 =

15

numerators.

2

6

x 3 =

5

15

x 3 =

1

5

x 5 =

+

3

x 5 =

15

11

15

1

2

x 2 =

6

12

x 2 =

1

3

x 3 =

+

4

x 3 =

12

5

Is this fraction in simplest form?

12

5

20

x 4 =

6

24

x 4 =

1

3

x 3 =

+

8

x 3 =

24

23

Is this fraction in simplest form?

24

2

6

x 3 =

3

9

x 3 =

1

1

x 1 =

+

9

x 1 =

9

7

Is this fraction in simplest form?

9

4

12

x 3 =

5

15

x 3 =

2

10

x 5 =

+

3

x 5 =

15

22

Is this fraction in simplest form?

15

1

R7

22

15)

1

22

1

15

15

7

7

1

15

Word Problem Practice:

Mrs. Walker graded 2/3 of the class’ math test and then stopped to take a phone call. When she returned, she graded 1/6 of the math test. What amount of the math test has she graded?

2/3 + 1/6 =

Mrs. Andrea is planning on having her art classes paint a picture. She will need 1/5 of a gallon of paint for her first period art class and 2/3 of a gallon for her second period art class. How much paint will be needed in all?

1/5 + 2/3 =

Shortcut for Finding the Least Common Denominator or Least Common Multiple

Check to see if the smaller denominator

divides evenly into the larger denominator.

If it does, use the larger denominator for

1

3 will divide evenly into 9,

so 9 is your LCD or LCM.

3

1

+

9

Use the short

cut to find the

Least Common

Denominator

(LCD).

1

2

8

1

+

8

8

Now find the

equivalent

fractions for

1/2 & 1/8.

1

2

8

x 4 =

1

+

8

x 1 =

8

Ask what do you multiply 2 by to get 8

and what do you multiply 8 by to get 8.

Since you are

writing equivalent

fractions, now

multiply the top

numbers by the

same number you

did in the

bottom.

1

x 4 =

2

8

x 4 =

1

x 1 =

+

8

x 1 =

8

1

4

Now multiply

across.

x 4 =

2

8

x 4 =

1

1

x 1 =

+

8

x 1 =

8

1

4

new

numerators.

x 4 =

2

8

x 4 =

1

1

x 1 =

+

8

x 1 =

8

5

8

Independent Practice:
• Math Book pg. 450 # 8-17
Subtracting Fractions With Unlike Denominators:

Follow the same 5 steps that you did to add fractions with unlike denominators.

• Step 1: Find the multiples of each denominator.
• Step 2: Determine the LCD.
• Step 3: Make equivalent fractions using the LCD as the new denominator.
• Step 4: Add/subtract the numerators and keep the denominators the same.
• Step 5: Check to make sure your answer is in simplest form.
Subtract these Fractions

Find the

common

Multiples for

5 and 2. Write

This number

denominator.

3

5

10

1

-

2

10

5 = 5, 10, 15, 20

2 = 2, 4, 6, 8, 10

Subtract these Fractions

what you

multiply the

bottom number

by to get 10.

3

5

10

x 2 =

1

-

2

10

x 5 =

Subtract these Fractions

Multiply the

top number

by the same

number you

did in the

bottom.

3

6

x 2 =

5

10

x 2 =

1

5

x 5 =

-

2

x 5 =

10

Subtract these Fractions

Now,

subtract

numerators.

3

6

x 2 =

5

10

x 2 =

1

5

x 5 =

-

2

x 5 =

10

1

Is this fraction in simplest form?

10

Subtract these Fractions

Don’t forget

to put your

simplest form!.

4

4

x 1 =

6

6

x 1 =

1

2

x 2 =

-

3

x 2 =

6

2

2

1

=

÷

6

2

3

Subtract these Fractions

5

20

x 4 =

6

24

x 4 =

1

3

x 3 =

-

8

x 3 =

24

17

Is this fraction in simplest form?

24

Subtract these Fractions

3

3

x 1 =

4

4

x 1 =

1

2

x 2 =

-

2

x 2 =

4

1

Is this fraction in simplest form?

4

Subtract these Fractions

3

6

x 2 =

5

10

x 2 =

1

5

x 5 =

-

2

10

x 5 =

1

Is this fraction in simplest form?

10

Subtract these Fractions

7

7

x 1 =

12

12

x 1 =

Is this fraction in simplest form?

1

3

x 3 =

-

4

12

x 3 =

4

4

1

=

÷

12

4

3

Word Problem Practice:

Johnny fed his two dogs. He fed the big dog 11/12 of a cup of dog food. He fed the little dog 1/4 of a cup of dog food. How much more food did the big dog get than the little dog?

1112 - 1/4 =

Susan is training for a 5K run. She ran 5/12 of a mile on Saturday and 5/6 of a mile on Sunday. What is the difference in the distance she ran?

5/12 - 5/6 =

Independent Practice:
• Math Book pg. 454-455 # 8-15 & 22-23