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1995-01-03
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êêèADDING FRACTIONS, INTERMEDIATE LEVEL
è In this section we will be looking at adding positive and/or nega-
tive fractions.ïYou recall in the Elementary Level that when you add
two positive fractions, you always get a positive answer in return.ïIn
this level we will have to consider the possibilities of adding one
positive fraction and one negative fraction, and adding two negative
fractions.ïWe will do this is two cases.ïIn case one we will look
at adding two positive fractions or two negative fractions. Case
two will involve adding two fractions with one positive and one nega-
tive.ïIn each of ç possible cases we will also consider adding
fractions with the same denominators and with different denominators.
That is a total of six combinations with which to deal.ïYou can see
why working with fractions is a difficult proposition.ïWe will ap-
proach this very systematically, however, by stating two rules that
cover all cases.ïFirst, we will describe the set of all fractions in
the following list.ïThis set of numbers is often called the Rational
Numbers.
êêêêïFractions
#êë ... -╩ï-╔è╚è╔è╩è╦è╠è═è╬ ...
êêè 1è1è1è1è1è1è1è1è1
#êë ... -╩ï-╔è╚è╔è╩è╦è╠è═è╬ ...
êêè 2è2è2è2è2è2è2è2è2
#êë ... -╦ï-╔è╚è╔è╩è╦è╠è═è╬ ...
êêè 3è3è3è3è3è3è3è3è3
êêêê.
êêêê.
êêêê.
Case 1)èThe first case involves adding two positive fractions or add-
ing two negative fractions.ïThese two problem types are covered by the
following rule for adding fractions with the same signs.
Rule 1)ïTo add two fractions that are both positive, just add them like
we did in the Elementary Level.ïTo add two fractions that are both
negative and have the same denominators, write down the common denomina-
tor once and add the two negative numbers in the numerators.ïTo add
two fractions that are both negative and have different denominators,
first write the fractions with common denominators.ïThen write down the
common denominator once and combine the two negative numbers in the nu-
merators.
Examples
ï1) Both fractions positive, and both fractions with the same denomi-
ënator.
êê 5è11êï5 + 11êï16êï4
#êê── + ──è =è ──────è =è ──è =è ─
êê12è12êè 12êè 12êï3
The answer can be left in this form or changed to the mixed number,
êêêêê1
#êêêêë1 ─.
êêêêê3
2) Both fractions positive, but different denominators.
êë1è2ê1è5è2è3ê 5è 6ê11
#êë─ + ─è=è─ ∙ ─ + ─ ∙ ─è=è── + ──è=è──
êë3è5ê3è5è5è3ê15è15ê15
3) Both fractions negative, and both with the same denominator.
êè -2è-3ê(-2) + (-3)ê-5êï5
#êè ── + ──è=è───────────è=è──èorï- ─
êë7è 7êë7êë 7êï7
4) Both fractions negative, but different denominators.
ï-1è-3ê-1 5è-3 4ê-5è-12ê(-5) + (-12)êï17
#ï── + ──è=è──∙─ + ──∙─è=è── + ───è=è────────────è=è- ──
è4è 5ê 4 5è 5 4ê20è 20êë20êê20
Case 2)ïThis case involves adding two fractions when one is negative
and the other is positive.ïThis problem type is covered by rule num-
ber two.
Rule 2)ïTo add two fractions when one is positive and the other is
negative, first write the problem so that the denominators are the same.
Then, write down the common denominator once and combine the positive
number and the negative number in the numerator.
Examples
1)ïOne fraction negative and one fraction positive, and both fractions
ëwith the same denominator.
êë2è-5ê2 + (-5)ê-3ê-1êè1
#ëa)ë─ + ──è=è────────è=è──è=è──èorè- ─
êë9è 9êè9êë9ê 3êè3
êë5è-1ê5 + (-1)ê 4
#ëb)ë─ + ──è=è────────è=è──
êë7è 7êè7êë7
2)ïOne fraction negative and one fraction positive, but with differ-
ëent denominators.
ë -2è2ê-2 3è2 5ê-6è10ê-6 + 10ê 4
#ïa)ï── + ─è=è──∙─ + ─∙─è=è── + ──è=è───────è=è──
ê5è3ê 5 3è3 5ê15è15êè15êï15
ë -4è1ê-4 2è1 5ê-8è 5ê-8 + 5ê -3
#ïb)ï── + ─è=è──∙─ + ─∙─è=è── + ──è=è──────è =è──
ê5è2ê 5 2è2 5ê10è10êè10êï10