Algebra Demystified 2nd Ed

16 alGebra De mystif ieD

being large or by having variables in them, it usually easier to use the LCD to

add or subtract fractions. The solution requires less simplifying, too.

In the following practice problems one of the denominators will be the LCD;

you only need to rewrite one fraction before computing the sum or difference.

PRACTICE

Find the sum or difference.

1.^1
8

1

2

+=

2.^2
3

5

12

−=

3.^4
5

1

20

+=

4.^7
30

2

15

−=

5.^5
24

5

6

+=

SOLUTIONS

1. 81 +=+⋅ 21 81 21 44 81 48 58









=+=

2.^23 −= 125 23 ⋅^4412512812512341









− =−==

3.^45 += 201 45 ⋅ 44 201 1620 201 1720









+=+=

4. 307 −= 152 307 −⋅ 152 22 307 304 303 101









=−==

5.^5
24

5

6

5

24

5

6

4

4

5

24

20

24

25

24

+= +⋅







=+ =

Finding the LCD

We have a couple of ways for finding the LCD. Take, for example, 121 + 149. We

could list the multiples of 12 and 14—the first number that appears on each

list is the LCD: 12, 24, 36, 48, 60, 72, 84 and 14, 28, 42, 56, 70, 84.

PRACTICE

Find the sum or difference.

✔SOLUTIONS