Transcript 3_9 Proportions TROUT 10

3.9 Proportions
• Goal to solve problems involving
proportions
Ratio and Proportion
A ratio of two quantities is a comparison,
often expressed as a fraction.
34
34 : 11
34  11
3.4
11
An equation that states that two ratios are
equal is called a proportion.
2 6

3 9
5 25

7 35
x 2

24 3
9 32

y 81
To Solve Proportions…
• Cross multiply and set their products = to
each other
• Ex: a = b
•
c
d
• so ad = cb
Example:
3 12

5 y
y = 20
Example:
x 7

4 6
14
x
3
Example:
 x  5 9
3

4x  20  27
4x  7
7
x
4
4
Proportions In Word Problems
• Comparing units
• Same units go in the numerator
• Same units go in the denominator
Strategies With Word Problems
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Read the problem at least two times
Write down the information you have
Identify what you are trying to find
Draw a picture
Set up an equation
Solve
Check
The property tax on a house is $8 per
$1000 of assessed value.
What is the tax on a house assessed
at $165,000?
8
t

1000 165,000
8 • 165,000 = 1000t
1000
1000
$1320  t
On a road map 3 inches represents
40 miles. If two cities are 10 inches
apart on the map, how many miles is
it from one to the other?
3"
10"

40mi
d
1
133 miles
3
Assignment
Page 150 (10-24 even and 25-38 all )