Download Cross Multiplication

Cross Multiplication
+
Objective:
We will learn to use cross
multiplication to solve a proportion.
We will use cross multiplication to
check whether two ratios form a
proportion.
+Property of Cross Products:
If two ratios form a proportion, the cross products are
equal. When 2 ratios have equal cross products, they
form a proportion.
Example 1:

Decide whether each pair of ratios forms a proportion.
You have to multiply the numerator with the denominator of the
other fraction; and vice versa.
105=
=105


3×35 = 5 ×21
105 = 105
If the two products are equal, they form a proportion.
When the cross products are not equal, the ratios do not form a proportion.
+ Cross multiplication, in a proportion,
is the product of one numerator and the
other denominator.
Example 1: 3 ? 5
9 = 15
Example 2:
3 ? 12
5 = 20
+
How to Solve a Proportion:
When solving a proportion you must cross multiply,
then find the missing variable.
 For example:
4 = H
Steps:

7
84



7×H = 4× 84
7H = 336
7 H = 336
7
7
H = 48
1. Write the cross products.
2. Multiply
3. To find the value of the variable, undo the
multiplication by dividing to both sides.
4. Divide
5. Write answer
(You have to think: The opposite of
multiplication is division, so I have to divide the
number that is with the variable and do the
same for both sides of the problem. Then I
cancel the opposites, and bring down what I
have left. )
+ Your Example:
Solve the proportion.
2= 3
4
N
X = 1
6
3
+
Homework:
In 1-5, answer yes or no, but you
must show your work.
Find the value of the variable by
using cross multiplication.
1.
3 ? 5
9 = 15
6. 3 = 11
X
20
2.
5 ? 2
8 = 3
3.
7 ? 25
9 = 27
4.
6 ? 10
8 = 15
5.
7 ? 20
9 = 27
7. 4 = 10
9
M
8.
Y = 3
10 5
9. 9 = T
1
4
10. 4 = X
7
84