Warm Up

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Warm Up
Problem of the Day
Lesson Presentation
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Warm Up Use the table to write each
ratio. 1. giraffes to monkeys 2. polar
bears to all bears 3. monkeys to all animals 4.
all animals to all bears
Brown bears 3
Giraffes 2
Monkeys 17
Polar Bears 4
217
47
1726
267
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A proportion is an equation that shows two
equivalent ratios.
Reading Math

Read the proportion as two is to one
as four is to two.
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Additional Example 1 Modeling Proportions Write
a proportion for the model.
First write the ratio of hearts to stars.

Next separate the hearts and stars into two equal
groups.
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Additional Example 1 Continued Now write the
ratio of hearts to stars in each group.

A proportion shown by the model is .
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Check It Out Example 1 Write a proportion for
the model.
First write the ratio of faces to moons.

Next separate the faces and moons into two equal
groups.
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Check It Out Example 1 Continued Now write the
ratio of faces to moons in each group.

A proportion shown by the model is .
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CROSS PRODUCTS CROSS PRODUCTS CROSS PRODUCTS CROSS PRODUCTS
Cross products in proportions are equal Cross products in proportions are equal Cross products in proportions are equal Cross products in proportions are equal

6 3
9 2
8 2
4 4
5 9
3 15
7 2
14 1
16 16
18 18
14 14
45 45
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Additional Example 2 Using Cross Products to
Complete Proportions Find the missing value in
the proportion.
Find the cross products.
6 n
The cross products are equal.
5 18
6n 90
n is multiplied by 6.
Divide both sides by 6 to undo the multiplication.

n 15
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Check It Out Example 2 Find the missing value in
the proportion.
Find the cross products.
5 n
The cross products are equal.
3 15
5n 45
n is multiplied by 5.
Divide both sides by 5 to undo the multiplication.

n 9
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Additional Example 3 Measurement
Application According to the label, 1 tablespoon
of plant fertilizer should be used per 6 gallons
of water. How many tablespoons of fertilizer
would you use for 4 gallons of water?
Let f be the amount of fertilizer for 4 gallons
of water.


Write a proportion.
6 f 1 4
The cross products are equal.
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Caution!
In a proportion, the units must be in the same
order in both ratios.

or

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Additional Example 3 Continued
f is multiplied by 6.
6f 4
Divide both sides by 6 to undo the multiplication.

Write your answer in simplest form.
f tbsp
You would use tbsp of fertilizer for 4
gallons of water.
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Check It Out Example 3 According to the label, 3
tablespoons of plant fertilizer should be used
per 9 gallons of water. How many tablespoons of
fertilizer would you use for 2 gallons of water?
Let f be the amount of fertilizer for 2 gallons
of water.


Write a proportion.
9 f 3 2
The cross products are equal.
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Check It Out Example 3 Continued
f is multiplied by 9.
9f 6
Divide both side by 9 to undo the multiplication.

Write your answer in simplest form.
f tbsp
You would use tbsp of fertilizer for 2
gallons of water.
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Lesson Quiz
1. Write a proportion for the model. Find the
missing value in each proportion. 2. 3.
4. The label on a bottle of salad
dressing states that there are 3 grams of fat per
tablespoon. If you use 3 tablespoons, how many
grams of fat would you be getting?

p 30
x 25
9 g