Algebra Demystified 2nd Ed

Chapter 1 FraCtions 15

PRACTICE

Find the sum or difference.

1.^5
6

1

5

−=

2.^1
3

7

8

+=

3.^5
7

1

9

−=

4. 143 += 21
   5.^3
   4

11

18

+=

SOLUTIONS

1.^5
6

1

5

5

6

5

5

1

5

6

6

25

30

6

30

19

30

−= ⋅











−⋅







=−=

2.^13 += ⋅ 87 31 88 87 33 248 2421 2924









+⋅







=+=

3.^5
7

1

9

5

7

9

9

1

9

7

7

45

63

7

63

38

63

−= ⋅











−⋅







=−=

4. 143 += 21 143 ⋅^2221141428628142028









+⋅







=+= ==^5

7

5.^34 += 1811 34 ⋅^1818111844547272449









+⋅







=+=^88

72

49

36

=

The Least Common Denominator (LCD)

Our goal is to add/subtract two fractions having the same denominator. In the

previous example problems and practice problems, we found a common

denominator. Now we will find the least common denominator (LCD). For

example in^13 + 61 , we could compute^13 += ⋅^16 ()^1366 +⋅() 61 33 =+ 186 183 == 189 21.

But we really only need to rewrite^13 :^13 += ⋅^16 ()^1322 +=+=^16626136 =^12.

While 18 is a common denominator in the above example, 6 is the smallest

common denominator. When denominators get more complicated, either by

✔SOLUTIONS

PRACTICE

Find the sum or difference.