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How do you subtract fractions with unlike denominators?
For example, what is - ?
-
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In this lesson you will learn how to subtract fractions with unlike denominators
by using fraction bars.
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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.
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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?
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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 .
=
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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
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A Common Misunderstanding
Students begin to subtract without forming a common denominator.
-
-≠
≠
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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.
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Core Lesson
-
+
Look at the denominator of the other fraction.
x Divide each part of the whole by 3
Multiply the pieces by 3=
=
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Core Lesson
-
+
Look at the denominator of the other fraction.
x Divide each part of the whole by 2
Multiply the pieces by 2=
= -
=
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Core Lesson
-
=
=
Now I can subtract.
= -
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Core LessonSammy says that - = , but
Trevan says it subtracts to leave .
It’s . It’s
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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.
![Page 14: How do you subtract fractions with unlike denominators?](https://reader036.vdocument.in/reader036/viewer/2022062301/56816276550346895dd2e749/html5/thumbnails/14.jpg)
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
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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
=
-
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Core Lesson
Now I can subtract.
-
=
= -
115 =
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Core Lesson
Trevan was correct because - =
It’s . Now I see it!
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In this lesson you have learned how to subtract fractions with
unlike denominators by using fraction bars.