Transcript Adding and Subtracting Fractions

Adding and Subtracting
Fractions
With different denominators
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.
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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.
L C D
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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.
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 × 4=4, 2 × 4=8, 3 ×
4=12, 4 × 4=16, 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.
Adding like Denominators
• Just simply add
the top 2
numbers of the
fractions
together since
they have
common
denominators.
Subtracting Like
Denominators
3/4 – 1/4 = 2/4
4/6 - 1/6 = 3/6
7/9 – 1/9 = 6/9
5/9 – 3/9 = 2/9
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
Adding Unlike Denominators
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 then add the
fractions!
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
HYPERLINKS
• Visit these websites below to learn more
about fractions
http://www.kidsolr.com/math/fractions.ht
ml
http://www.aaamath.com/B/fra.htm
http://www.aaamath.com/fra410aaddfractld.html
GAME
• Go to
http://www.aaamath.com/fr
a66l-subfracud.html to
play the online fraction
game!
• Click start and then solve
the subtraction problem,
and then report your score
at the end!
YOU DID IT! NOW YOU
ALL KNOW YOUR
FRACTIONS!