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Objective
Know how to convert two fractions into LCD form.
Example
Convert the following fractions into LCD form.
5
9
-
1
2
Answer:
10
18
-
9
18
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Explanation
Lowest Common Denominator (LCD)
Lowest: smallest
Common: for both
Denominator: the bottom number in a fraction
The bottom line is, you can't add or subtract two fractions if they have different denominators. Putting fractions. Putting fractions into LCD form solves this problem.
5
9
-
1
2
If you can't tell what the LCD is just by looking, you can create a table.
Put you two denominators on top. Then, start listing multiples (times 1, times 2, times 3, etc.)
Once you see the same number come up, you can stop. This is your LCD
For 9, the LCD was the 2nd multiple (9x2).
For 2, the LCD was the 9th multiple (2x9).
So, for 9, multiply the numerator and denominator by 2
5 x 2
9 x 2
=
10
18
For 2, multiply the numerator and denominator by 9
1 x 9
2 x 9
=
9
18
That's all. Notice that if both denominators aren't now the LCD, you did something wrong.
10
18
-
9
18
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Examples
Example 1
Convert the following fractions into LCD form
2
6
+
7
8
2 x 4
6 x 4
7 x 3
8 x 3
=
8
24
=
21
24
Answer:
8
24
+
21
24
~~~~~~~~~~~~~~~~~~~
Example 2
Convert the following fractions into LCD form
2
4
+
7
8
2 x 2
4 x 2
7 x 1
8 x 1
=
4
8
=
7
8
Answer:
4
8
+
7
8
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Practice your skills
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Ready?
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