1. Lesson 5-1
Using
Proportions

2. A ratio is a comparison of two numbers such as 4:5
A ratio is a comparison of two numbers such as 4:5. When writing a ratio, always express it in simplest form. A B C D
3.6 6 8
4.8 10
What is the ratio of segment AB to segment CB? AB CB = 10 6
Now try to reduce the fraction. Once reduced this is
the ratio between the two sides. 10 6 = 5 3
Therefore, the ratio of
segment AB to segment CB is 5:3.
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3. A baseball player goes to bat 348 times and gets 107 hits.
What is the player’s batting average?
To find a baseball player’s batting average a ratio is first set up.
The ratio compares the number of hits to the number of times
he goes to bat. In this problem, the ratio would be 107 to 348
or in fraction form 107/ Now convert this fraction to a decimal
by dividing the denominator into the numerator and then rounding
to three decimal places.
Ratio:
107
348
Decimal:
107 / 348 = 0.307
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The baseball player’s batting average is which means roughly he is getting one hit every three times at bat.

4. a b c d Proportion: an equation that states that two ratios are equal.=
Ratio
Ratio
Proportion
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5. = Solving a proportion:
There are two common ways to describe the method:
Cross-multiply
Use cross-products a b = c d
cross products
a • d =
b • c
ad = bc
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Now solve the resulting equation.

6. = Solving a proportion: x Example 5 6 8 = 8 • x 6 • 5 8x = 30 8x = 30
x = 3.75
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7. Proportions The sides of these two triangles are proportional,
Because all 3 ratios of corresponding sides are
equal.
Not drawn to scale! X 2 F e t
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356 yards
84 yards
How tall is the cactus? (Note the different units.)

8. Review Terms: Ratio Proportion Cross product Cross-multiply
1. A ratio is _____________________________________. In the example
above ______ and ______ are ratios.
2. A proportion is _________________________________________
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3. To solve the above equation, take the _________________________ or ______________________.