chapter 7 section 3. objectives 1 copyright © 2012, 2008, 2004 pearson education, inc. least common...
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Chapter 7 Section 3
Objectives
1
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Least Common Denominators
Find the least common denominator for a group of fractions.
Write equivalent rational expressions.
7.3
2
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Objective 1
Find the least common denominator for a group of fractions.
Slide 7.3-3
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Find the least common denominator for a group of fractions.
Adding or subtracting rational expressions often requires a least
common denominator (LCD), the simplest expression that is
divisible by all of the denominators in all of the expressions. For
example, the least common denominator for the fractions and
is 36, because 36 is the smallest positive number divisible by both 9
and 12.
2
9
5
12
We can often find least common denominators by inspection. For
example, the LCD for and is 6m. In other cases, we find the
LCD by a procedure similar to that used in Section 6.1 for finding the
greatest common factor.
1
6
2
3m
Slide 7.3-4
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Finding the Least Common Denominator (LCD)
Step 1: Factor each denominator into prime factors.
Step 2: List each different denominator factor the greatest number of times it appears in any of the denominators.
Step 3: Multiply the denominator factors from Step 2 to get the LCD.
When each denominator is factored into prime factors, every prime factor must be a factor of the least common denominator.
Slide 7.3-5
Find the least common denominator for a group of fractions. (cont’d)
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Find the LCD for each pair of fractions.
Solution:
7 1,
10 25
4 6
4 11,
8 12m m
10 2 5
4 48 2 2 2m m
25 5 5
6 612 2 2 3m m
432 m 622 3 m
52 25
2LCD 5 2
3 63D 2LC m
50
624m
Slide 7.3-6
EXAMPLE 1 Finding the LCD
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Find the LCD for
Solution:
3 5
4 5 and .
16 9m n m
3 316 2 2 2 2m n m n 5 59 3 3m m
342 nm 2 53 m
4 2 52 3LCD m n 5144m n
When finding the LCD, use each factor the greatest number of times it appears in any single denominator, not the total number of times it appears.
Slide 7.3-7
EXAMPLE 2 Finding the LCD
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Solution:
4 1,
1 1x x
Find the LCD for the fractions in each list.
2 2
6 3 1,
4 16
x
x x x
4x x
4 4x x
LCD 4 4x x x
4x x
44 xx
Either x − 1 or 1 − x, since they are opposite expressions.
Slide 7.3-8
EXAMPLE 3 Finding LCDs
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Objective 2
Write equivalent rational expressions.
Slide 7.3-9
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Write equivalent rational expressions.
Writing A Rational Expression with a Specified Denominator
Step 1: Factor both denominators.
Slide 7.3-10
Step 2: Decide what factor (s) the denominator must be multiplied by in order to equal the specified denominator.
Step 3: Multiply the rational expression by the factor divided by itself. (That is, multiply by 1.)
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
Rewrite each rational expression with the indicated denominator.
Solution:
3
4
93
4 9
3
4 9
?
4
242
30
k
k
3 ?
4 36
7 ?
5 65
k
k
7 7
5 5
6
6
k k k
k 7 ?
5 30
k
k
27
36
Slide 7.3-11
EXAMPLE 4 Writing Equivalent Rational Expressions
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Rewrite each rational expression with the indicated denominator.
Solution:
9 ?
2 5 6 15a a
2
5 1 ?
2 2 1
k
k k k k k
9 ?
32 5 2 5a a
9 9
2 5 2
3
35a a
27
6 15a
1
5 1 ?
2 2
k
k k kk k
5 1 5 1 1
2 12
k k
k k k k
k
k
5 1 1
2 1
k k
k k k
Slide 7.3-12
EXAMPLE 5 Writing Equivalent Rational Expressions
Copyright © 2012, 2008, 2004 Pearson Education, Inc.
HL # 7.3Book Beginning AlgebraPage 439 Exercises 38, 39, 40, 41, 42, 43, 44.Page 440 Exercises 55, 56, 57, 58, 64, 65, 67.