How do you subtract fractions with unlike denominators?

For example, what is - ?

-

In this lesson you will learn how to subtract fractions with unlike denominators

by using fraction bars.

Let’s Review

We know that fractions need to have the same denominator to subtract them.

I remember my teacher called this a common denominator.

I can use equivalent fractions to

make common denominators.

Let’s Review

We know that equivalent fractions show equal amounts.

= = = = =

This could go on forever!

How do I decide which equivalent fraction to use?

Let’s Review

When one denominator is a divisor of the other, only one fraction has to be changed.

-

-

2 is a divisor of 8 so I will make an

equivalent fraction for .

=

Let’s Review

When both denominators are multiples of 10, only one fraction has to be changed.

- =51100

I know that is equivalent to

70100

19100

A Common Misunderstanding

Students begin to subtract without forming a common denominator.

-

-≠

≠

Core LessonWhat is - ?

-

One denominator is not a divisor of

the other.

Neither denominator is a multiple of 10.

How do I decide which equivalent

fractions to form?

I can multiply any number by 1 without changing its value.

Core Lesson

-

+

Look at the denominator of the other fraction.

x Divide each part of the whole by 3

Multiply the pieces by 3=

=

Core Lesson

-

+

Look at the denominator of the other fraction.

x Divide each part of the whole by 2

Multiply the pieces by 2=

= -

=

Core Lesson

-

=

=

Now I can subtract.

= -

Core LessonSammy says that - = , but

Trevan says it subtracts to leave .

It’s . It’s

Core Lesson

What is - ?

-

One denominator is

not a multiple of the other.

Neither denominator is a multiple of 10.

How do I decide which equivalent

fractions to form?

I can multiply any number by 1 without changing its value.

Core Lesson

-

Look at the denominator of the other fraction.

x Divide each part of the

whole by 3

Multiply the pieces by 3=

- = 2

5

Core Lesson

Look at the denominator of the other fraction.

x Divide each part of the whole by 5

Multiply the pieces by 5= = -

-13

=

-

Core Lesson

Now I can subtract.

-

=

= -

115 =

Core Lesson

Trevan was correct because - =

It’s . Now I see it!

In this lesson you have learned how to subtract fractions with

unlike denominators by using fraction bars.