# Adding and Subtracting Fractions - PowerPoint PPT Presentation

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### Adding and Subtracting Fractions With different denominators – PowerPoint PPT presentation

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1
• With different denominators

2
Different Denominators
• It's easy to add and subtract like fractions, or
fractions with the same denominator.
• You just add or subtract the numerators and keep
the same denominator.
• To do this, you need to know how to find the
least common denominator.
• In an earlier lesson, you learned how to
simplify, or reduce, a fraction by finding an
equivalent, or equal, fraction where the
numerator and denominator have no common factors.
• To do this, you divided the numerator and
denominator by their greatest common factor.
• Next we will learn that you can also multiply the
numerator and denominator by the same factor to
make equivalent fractions.

3
L C D
• Since only like fractions can be added or
subtracted, we first have to convert unlike
fractions to equivalent like fractions.
• We want to find the smallest, or least, common
denominator, because working with smaller numbers
makes our calculations easier.
• The least common denominator, or LCD, of two
fractions is the smallest number that can be
divided by both denominators.

4
Methods for Finding LCD
• The first method is to simply start writing all
the multiples of both denominators, beginning
with the numbers themselves. Here's an example of
this method. Multiples of 4 are 4, 8, 12, 16, and
so forth (because 1 44, 2 48, 3 412, 4
416, etc.). The multiples of 6 are 6,
12,--that's the number we're looking for, 12,
because it's the first one that appears in both
lists of multiples. It's the least common
multiple, which we'll use as our least common
denominator.

5
• Just simply add the top 2 numbers of the
fractions together since they have common
denominators.

6
Subtracting Like Denominators
• Subtracting like denominators is just as easy as
adding them! All you have to do is subtract the
numerators and keep the same common denominator

3/4 1/4 2/4 4/6 - 1/6 3/6 7/9 1/9
6/9 5/9 3/9 2/9
7
Find the least common denominator between 12 and
16 which is 48. Then find out what number
times 12 is equal to 48 and do the same for 16
(3). Multiply 3 by both of the numerators, and
8
Subtracting Unlike Denominators
• The least common denominator between 3 and 12 is
12
• Find what number times 3 is equal to 12 and then
multiply that by 1
• Then simply subtract the numerator

9
fractions
• http//www.kidsolr.com/math/fractions.html
• http//www.aaamath.com/B/fra.htm