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HELP1A
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1995-03-22
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êêè ADDING FRACTIONS, ELEMENTARY LEVEL
è In this section we will be looking at adding positive fractions.ïWe
will do this by considering two cases.ïThe first case will involve add-
ing positive fractions that have the same denominators.ïCase two, a
more general problem type, includes adding positive fractions that have
different denominators.ïIn both cases when you add two positive frac-
tions, you always get a positive fraction as the answer.ïThe positive
fractions are described in the list below.ïThe number at the beginning
of each row is the same as the number zero.ïActually, we are looking at
the positive fractions and the number zero.ïThese numbers are often
referred to as the non-negative fractions, but it seems more straight
forward to just say "the positive fractions."
êêêë Positive Fractions
#êêê ╚è╔è╩è╦è╠è═è╬ ...
êêê 1è1è1è1è1è1è1
#êêê ╚è╔è╩è╦è╠è═è╬ ...
êêê 2è2è2è2è2è2è2
#êêê ╚è╔è╩è╦è╠è═è╬ ...
êêê 3è3è3è3è3è3è3
êêêè.
êêêè.
êêêè.
Case 1)èAdding Positive Fractions With the Same Denominators
è To add positive fractions that have the same denominators, you
should write down the common denominator once then add the numerators.
Example 1)
è To add the fractions 5/12, 7/12, and 1/12, you should write down the
common denominator once then add the numerators.
êê 5è 7è 1êï5 + 7 + 1êï13
#êê── + ── + ──è =è ─────────è =è ──
êê12è12è12êë12êë 12
The answer can be left in this form or changed to the mixed number,
êêêêê 1
#êêêêë1 ──.
êêêêê12
Case 2)ïAdding Positive Fractions with Different Denominators
è To add positive fractions that have different denominators, it is
first necessary to express the denominators in prime factored form.ïTo
write a denominator in prime factored form, you should break it down in-
to products of prime numbers.ïThe prime numbers are described in the
following list.
êêè 2, 3, 5, 7, 11, 13, 17, 19,...
Each of ç numbers has the property that the only factors of each
number are "1" and the number itself.ïOnce the denominators are in
prime factored form, you can multiply the top and bottom of individual
fractions by missing factors to make all of the denominators the same.
Then, write down the common denominator once and add the numerators.
Example 2)ïExpress the number, 6, in prime factored form.ïSince the
number, 6, can be factored into the product of the two prime numbers
2 and 3, the prime factorization of 6 is 2∙3.
Example 3)ïExpress the number, 18, in prime factored form.ïYou should
start with the smallest prime number, 2, and see if it divides evenly
into 18.ïSince 2 goes into 18 nine times, you can express 18 as 2∙9.
Also, since the next smallest prime number, 3, divides evenly into 9,
18 can be expressed as 2∙3∙3.ïSince ç factors are all prime num-
bers, the prime factorization of 18 is 2∙3∙3.
Example 4)
è To add the fractions,ï1/14ïandï5/7 , it is first necessary
to express the denominators in prime factored form.
êêêè1ë5êè1ë 5
#êêêï──ï+ï─è =è ───ï+ï─
êêêï14ë7êï2∙7ë7
At this point you can see that the second fraction is missing a factor
of "2" in its denominator.ïIt is necessary to multiply both the top
and the bottom of this fraction by "2".
êêê1ë 5êè1ë 5è2
#êêë ───ï+ï─è =è ───ï+ï─ ∙ ─
êêë 2∙7ë7êï2∙7ë7è2
Now, both denominators have the same factors, and you can multiply to
simplify the form of the problem.
êêê1ë 5è2êè1ë10
#êêë ───ï+ï─ ∙ ─è =è ──ï+ï──
êêë 2∙7ë7è2êï14ë14
Since the two fractions have the same denominators, you can write down
the denominator once and add the numerators like we did in Example 1.
êêê 1ë10êï1 + 10êï11
#êêê──ï+ï──è =è ──────è =è ──
êêê14ë14êè 14êè 14
Thus, the sum ofï1/14ïandï5/7ïisï11/14.
è Another way to add fractions is to add them in a column.ïLeon the
Fraction Wizard prefers to use the method in the above examples, and his
method should be considered to be correct and general in the sense that
it always works no matter how big the numbers.èMany people, however,
prefer to add fractions in a column.ïIt is still necessary to find the
least common denominator when you add fractions in a column, and it is
perfectly alright to just write down the least common denominator if you
can identify it by inspection.ïYou can always go back to the prime fac-
torization method if the numbers are too large to identify the least
common denominator by inspection.
Example 5)
êêïAdd the fractions 1/14 and 5/7 in a column.
First, you should identify the least common denominator and write it
down next to the original problem.ïThen, you can find the missing nu-
merators by dividing and multiplying.ïFinally, the resulting fractions
should be added.
êêêï1êêê1
#êêê ──è =è ──è =è ──
êêê 14êï14êï14
êêêï5êêë 10
#êêë+è─è =è ──è =è ──
êêêï7êï14êï14
#êêê────ê────ê────
êêêêêêï11
#êêêêêêï──
êêêêêêï14