2. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Cross Multiplication
3. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
Cross Multiplication
4. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
Cross Multiplication
5. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
Cross Multiplication
6. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
7. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
8
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
8. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
8
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
{ 16,
9. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
8 3
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
{ 16,
10. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
8 3
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
{ 16, 15,
11. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
8 3 62
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
{ 16, 15, 14, 18}
12. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
8 3 62
{ 16, 15, 14, 18}
c. List the fractions from
the largest to the smallest.
From the whole numbers,
listing the fractions from the
largest to the smallest:
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
13. The importance of the LCD is that the LCD is the smallest,
hence the easiest number to “multiply” all the fractions in
question into whole numbers.
Example A. a. Find the LCD
of the following list of fractions,
{ 2/3, 5/8, 7/12, 3/4}
b. Multiply the LCD to each
fraction to change the list
into a whole-number list.
List the multiples of 12
(from 7/12): 12, 24, 36,..
we find that the LCD is 24.
{ 2/3, 5/8, 7/12, 3/4} x 24
8 3 62
{ 16, 15, 14, 18}
c. List the fractions from
the largest to the smallest.
From the whole numbers,
listing the fractions from the
largest to the smallest: 3/4, 2/3, 5/8, 7/12
1418 16 15
{ 2/3, 5/8, 7/12, 3/4} x 24
Cross Multiplication
14. With two fractions, here is an easier and useful procedure
for clearing their denominators.
Cross Multiplication
Cross Multiplication
15. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below,
Cross Multiplication
16. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators,
Cross Multiplication
17. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators,
Cross Multiplication
bdbd
18. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators,
Cross Multiplication
bdbd
ad
19. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators,
Cross Multiplication
bdbd
ad bc
20. With two fractions, here is an easier and useful procedure
for clearing their denominators.
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators, yield the same outcome as:
taking the denominators and multiply them diagonally across.
Cross Multiplication
a
b
c
d
bdbd
ad bc
21. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators, yield the same outcome as:
taking the denominators and multiply them diagonally across.
ad bc
Cross Multiplication
(cross–multiplication)
22. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators, yield the same outcome as:
taking the denominators and multiply them diagonally across.
ad bc
Be sure the denominators cross over and up
so the outcomes correspond to the fractions.
Cross Multiplication
(cross–multiplication)
23. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators, yield the same outcome as:
taking the denominators and multiply them diagonally across.
ad bc
Be sure the denominators cross over and up
so the outcomes correspond to the fractions.
Do not cross downward as shown here!!
a
b
c
d
adbc
Cross Multiplication
(cross–multiplication)
24. With two fractions, here is an easier and useful procedure
for clearing their denominators.
a
b
c
d
Cross Multiplication
Given two fractions as shown below, multiplying to each fraction
by the common denominator bd (which may not be the LCD),
to clear the denominators, yield the same outcome as:
taking the denominators and multiply them diagonally across.
ad bc
Be sure the denominators cross over and up
so the outcomes correspond to the fractions.
Do not cross downward as shown here!!
a
b
c
d
adbc
Cross Multiplication
(The results are out of order!)
(cross–multiplication)
25. Here are some operations where we may cross multiply.
Cross Multiplication
26. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
Cross Multiplication
27. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2,
Cross Multiplication
28. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour.
Cross Multiplication
29. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3.
Cross Multiplication
30. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing.
Cross Multiplication
31. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing. It’s better to
cross multiply to rewrite this ratio in whole numbers.
Cross Multiplication
32. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing. It’s better to
cross multiply to rewrite this ratio in whole numbers.
Example B.
rewrite a recipe that calls for the fractional ratio of 3/4 cup
sugar to 2/3 cup of flour into ratio of whole numbers.
Cross Multiplication
33. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing. It’s better to
cross multiply to rewrite this ratio in whole numbers.
Example B.
rewrite a recipe that calls for the fractional ratio of 3/4 cup
sugar to 2/3 cup of flour into ratio of whole numbers.
Write 3/4 cup of sugar as and 2/3 cup of flour as3
4 S
2
3 F.
Cross Multiplication
34. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing. It’s better to
cross multiply to rewrite this ratio in whole numbers.
Example B.
rewrite a recipe that calls for the fractional ratio of 3/4 cup
sugar to 2/3 cup of flour into ratio of whole numbers.
Write 3/4 cup of sugar as and 2/3 cup of flour as3
4 S
2
3 F.
We have the ratio 3
4
S : 2
3
F
Cross Multiplication
35. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing. It’s better to
cross multiply to rewrite this ratio in whole numbers.
Example B.
rewrite a recipe that calls for the fractional ratio of 3/4 cup
sugar to 2/3 cup of flour into ratio of whole numbers.
Write 3/4 cup of sugar as and 2/3 cup of flour as3
4 S
2
3 F.
We have the ratio 3
4
S : 2
3
F cross multiply we’ve 9S : 8F.
Cross Multiplication
36. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing. It’s better to
cross multiply to rewrite this ratio in whole numbers.
Example B.
rewrite a recipe that calls for the fractional ratio of 3/4 cup
sugar to 2/3 cup of flour into ratio of whole numbers.
Write 3/4 cup of sugar as and 2/3 cup of flour as3
4 S
2
3 F.
We have the ratio 3
4
S : 2
3
F cross multiply we’ve 9S : 8F.
Hence in integers, the ratio is 9 : 8 for sugar : flour.
Cross Multiplication
37. Here are some operations where we may cross multiply.
Rephrasing Fractional Ratios
If a cookie recipe calls for 3 cups of sugar and 2 cups of flour,
we say the ratio of sugar to flour is 3 to 2, and it’s written as
3 : 2 for sugar : flour. Or we said the ratio of flour : sugar is
2 : 3. For most people a recipe that calls for the fractional ratio
of 3/4 cup sugar to 2/3 cup of flour is confusing. It’s better to
cross multiply to rewrite this ratio in whole numbers.
Example B.
rewrite a recipe that calls for the fractional ratio of 3/4 cup
sugar to 2/3 cup of flour into ratio of whole numbers.
Write 3/4 cup of sugar as and 2/3 cup of flour as3
4 S
2
3 F.
We have the ratio 3
4
S : 2
3
F cross multiply we’ve 9S : 8F.
Hence in integers, the ratio is 9 : 8 for sugar : flour.
Cross Multiplication
Remark: A ratio such as 8 : 4 should be simplified to 2 : 1.
39. Cross–Multiplication Test for Comparing Two Fractions
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller.
40. Cross–Multiplication Test for Comparing Two Fractions
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
41. Cross–Multiplication Test for Comparing Two Fractions
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
42. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
43. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45
we get
44. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
45. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
3
5
5
8
46. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
Cross– multiply 3
5
5
8
47. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
Cross– multiply 3
5
5
8
24 25
we get
48. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
Cross– multiply 3
5
5
8
24 25
we get
moreless
49. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
Cross– multiply 3
5
5
8
24 25
Hence 3
5
5
8
is less than
we get
moreless
.
50. Cross–Multiplication Test for Comparing Two Fractions
Hence cross– multiply
Cross Multiplication
When comparing two fractions to see which is larger and
which is smaller. Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
Cross– multiply 3
5
5
8
24 25
Hence 3
5
5
8
is less than
we get
moreless
.
(Which is more
7
11
9
14
or ? Do it by inspection.)
53. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions
a
b
c
d
±
Cross Multiplication
54. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions
a
b
c
d± =
ad ±bc
Cross Multiplication
55. Cross–Multiplication for Addition or Subtraction
a
b
c
d± =
ad ±bc
Cross Multiplication
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
56. Cross–Multiplication for Addition or Subtraction
a
b
c
d± =
ad ±bc
bd
Cross Multiplication
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
57. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
± =
ad ±bc
bd
Cross Multiplication
58. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
–a.
Cross Multiplication
59. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
–a.
Cross Multiplication
60. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
a.
Cross Multiplication
61. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
Cross Multiplication
62. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
–b.
Cross Multiplication
63. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
–b.
Cross Multiplication
64. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
– =
5*12 – 9*5
9*12
b.
Cross Multiplication
65. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
– =
5*12 – 9*5
9*12
15
108
=b.
Cross Multiplication
66. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
– =
5*12 – 9*5
9*12
15
108
=b. 5
36
=
Cross Multiplication
67. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
– =
5*12 – 9*5
9*12
15
108
=b. 5
36
=
Cross Multiplication
In a. the LCD = 30 = 6*5 so the crossing method is the same as
the Multiplier Method.
68. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
– =
5*12 – 9*5
9*12
15
108
=b. 5
36
=
Cross Multiplication
In a. the LCD = 30 = 6*5 so the crossing method is the same as
the Multiplier Method. However in b. the crossing method
yielded an answer that needed to be reduced.
69. Cross–Multiplication for Addition or Subtraction
We may cross multiply to add or subtract two fractions with
the product of the denominators as the common denominator.
a
b
c
d
Afterwards we reduce if necessary for the simplified answer.
Example C. Calculate
± =
ad ±bc
bd
3
5
5
6
– =
5*5 – 6*3
6*5
7
30
=a.
5
12
5
9
– =
5*12 – 9*5
9*12
15
108
=b. 5
36
=
Cross Multiplication
In a. the LCD = 30 = 6*5 so the crossing method is the same as
the Multiplier Method. However in b. the crossing method
yielded an answer that needed to be reduced. we need both
methods.
70. Comparing Multiple Fractions
Hence cross– multiply
Cross Multiplication
The reason that Cross–multiply them, the side with the larger
product corresponds to the larger fraction.
In particular, if the cross multiplication products are the same
then the fraction are the same.
3
5
9
15
=45 45 so
3
5
9
15
=
we get
Cross– multiply 3
5
5
8
24 25
Hence 3
5
5
8
is less than
we get
moreless
.
(Which is more
7
11
9
14
or ? Do it by inspection.)
72. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
73. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways.
74. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct.
75. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
76. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer.
77. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer. For example, in part b. above,
we obtain an answer via the crossing method.
78. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer. For example, in part b. above,
we obtain an answer via the crossing method. Let’s cross
check the first answer using the Multiplier Method.
5
12
5
9
–
79. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer. For example, in part b. above,
we obtain an answer via the crossing method. Let’s cross
check the first answer using the Multiplier Method.
Since the LCD = 36, we multiply and divide by 36.
5
12
5
9
–
80. The Double Check Strategy
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
Cross Multiplication
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer. For example, in part b. above,
we obtain an answer via the crossing method. Let’s cross
check the first answer using the Multiplier Method.
Since the LCD = 36, we multiply and divide by 36.
5
12
5
9
–( ( *36 / 36
81. The Double Check Strategy
Cross Multiplication
5
12
5
9
–( ( *36 / 36
34
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer. For example, in part b. above,
we obtain an answer via the crossing method. Let’s cross
check the first answer using the Multiplier Method.
Since the LCD = 36, we multiply and divide by 36.
82. The Double Check Strategy
Cross Multiplication
5
12
5
9
–( ( *36 / 36
= (5*4 – 5*3) / 36 = 5/36
34
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer. For example, in part b. above,
we obtain an answer via the crossing method. Let’s cross
check the first answer using the Multiplier Method.
Since the LCD = 36, we multiply and divide by 36.
83. The Double Check Strategy
Cross Multiplication
5
12
5
9
–( ( *36 / 36
= (5*4 – 5*3) / 36 = 5/36
34
One of the most difficult thing to do in mathematics is to know
that a mistake had taken place and to locate the mistake.
The Double Check is to cross check an answer by doing a
problem two different ways. If both methods yielded the same
answer then the answer is likely to be correct. If two answers
are different then we have to clarify the mistake.
When we + or – fractions, we can use the above two methods
to cross check an answer. For example, in part b. above,
we obtain an answer via the crossing method. Let’s cross
check the first answer using the Multiplier Method.
Since the LCD = 36, we multiply and divide by 36.
This is the same as before hence it’s very likely to be correct.
84. Cross Multiplication
Comments
* The Double Check Strategy is an important tool for learning.
It reassures us if we’re heading in the right direction. It warns
us that a mistake had occurred so we should back track and
locate the mistake.
85. Cross Multiplication
Comments
* The Double Check Strategy is an important tool for learning.
It reassures us if we’re heading in the right direction. It warns
us that a mistake had occurred so we should back track and
locate the mistake.
Use this Double Check Strategy for learning!
86. Cross Multiplication
* The Multiplier Method and the Cross Multiplication Method
are two methods to double check addition and subtraction of
small number of fractions.
Comments
* The Double Check Strategy is an important tool for learning.
It reassures us if we’re heading in the right direction. It warns
us that a mistake had occurred so we should back track and
locate the mistake.
Use this Double Check Strategy for learning!
87. Cross Multiplication
* The Multiplier Method and the Cross Multiplication Method
are two methods to double check addition and subtraction of
small number of fractions. These two methods generalize to
addition and subtraction of fractional (rational) formulas in
later topics.
Comments
* The Double Check Strategy is an important tool for learning.
It reassures us if we’re heading in the right direction. It warns
us that a mistake had occurred so we should back track and
locate the mistake.
Use this Double Check Strategy for learning!
88. Cross Multiplication
* The Multiplier Method and the Cross Multiplication Method
are two methods to double check addition and subtraction of
small number of fractions. These two methods generalize to
addition and subtraction of fractional (rational) formulas in
later topics. Each method leads to various ways of handling
various fractional algebra problems where each way has its
own advantages and disadvantage.
Comments
* The Double Check Strategy is an important tool for learning.
It reassures us if we’re heading in the right direction. It warns
us that a mistake had occurred so we should back track and
locate the mistake.
Use this Double Check Strategy for learning!
89. Cross Multiplication
* The Multiplier Method and the Cross Multiplication Method
are two methods to double check addition and subtraction of
small number of fractions. These two methods generalize to
addition and subtraction of fractional (rational) formulas in
later topics. Each method leads to various ways of handling
various fractional algebra problems where each way has its
own advantages and disadvantage.
We use both methods through out this database.
Comments
* The Double Check Strategy is an important tool for learning.
It reassures us if we’re heading in the right direction. It warns
us that a mistake had occurred so we should back track and
locate the mistake.
Use this Double Check Strategy for learning!
90. Ex. Restate the following ratios in integers.
9. In a market, ¾ of an apple may be traded with ½ a pear.
Restate this using integers.
1
2
1
3
:1. 2. 3. 4.2
3
1
2:
3
4
1
3
:
2
3
3
4
:
3
5
1
2:5. 6. 7. 8.1
6
1
7
:
3
5
4
7:
5
2
7
4:
Determine which fraction is more and which is less.
2
3
3
4
,10. 11. 12. 13.4
5
3
4
,
4
7
3
5
,
5
6
4
5
,
5
9
4
7
,14. 15. 16. 17.7
10
2
3
,
5
12
3
7
,
13
8
8
5
,
1
2
1
3
+18. 19. 20. 21.1
2
1
3
–
2
3
3
2
+
3
4
2
5
+
5
6
4
7
–22. 23. 24. 25.7
10
2
5
–
5
11
3
4
+
5
9
7
15
–
Cross Multiplication
C. Use cross–multiplication to combine the fractions.