Warm Up

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Warm Up
Problem of the Day
Lesson Presentation
Lesson Quizzes
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Warm Up Simplify. 1. 82 2. 122 3. 6.22 4.
7.52
64
144
38.44
56.25
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Problem of the Day A 16 in. pizza sells for
11.99. A 10 in. pizza sells for 5.99. Which
size gives you more pizza per penny? Explain.
The larger pizza you get more than twice as much
(64? in2 instead of 25? in2) for about twice the
price.
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I can find the area of circles.
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A circle can be cut into equal-sized sectors and
arranged to resemble a parallelogram. The height
h of the parallelogram is equal to the radius r
of the circle, and the base b of the
parallelogram is equal to one-half the
circumference C of the circle.
So the area of the parallelogram can be written
as
1 2
A bh, or A
Cr.
1 2
Since C 2?r, A
(2?r)r ?r2.
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(No Transcript)
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Additional Example 1A Finding the Area of a
Circle
Find the area of the circle to the nearest tenth.
Use 3.14 for ?.
A ?r2
Use the formula.
7 cm
A ? 3.14 72
Substitute 7 for r.
A ? 3.14 49
Evaluate the power.
Multiply.
A ? 153.86
The area of the circle is about 153.9 cm2.
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Additional Example 1B Finding the Area of a
Circle
Find the area of the circle to the nearest tenth.
Use 3.14 for ?.
A ?r2
Use the formula.

A ? 3.14 92
18 ft
Substitute 9 for r.
A ? 3.14 81
Evaluate the power.
Multiply.
A ? 254.34
The area of the circle is about 254.3 ft2.
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Check It Out Example 1A
Find the area of the circle to the nearest tenth.
Use 3.14 for ?.
A ?r2
Use the formula.
A ? 3.14 102
10 cm
Substitute 10 for r.
A ? 3.14 100
Evaluate the power.
Multiply.
A ? 314
The area of the circle is about 314 cm2.
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Check It Out Example 1B
Find the area of the circle to the nearest tenth.
Use 3.14 for ?.
A ?r2
Use the formula.

12 ft
A ? 3.14 62
Substitute 6 for r.
A ? 3.14 36
Evaluate the power.
Multiply.
A ? 113.04
The area of the circle is about 113.0 ft2.
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Additional Example 2 Application
Park employees are fitting a top over a circular
drain in the park. If the radius of the drain is
14 inches, what is the area of the top that will
cover the drain? Use
22 7
for ?.
A ? r2
Use the formula for the area of a circle.
22 7
A ?
142
Substitute. Use 14 for r.
28
22 7
196
A ?

Evaluate the power.
1
A ? 22 28
Multiply.
A ? 616
The area of the top that will cover the drain is
about 616 in2.
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Check It Out Example 2
Albert was designing a cover for a spa. If the
radius of the spa is 7 ft, what is the area of
the cover that will be made? Use
22 7
for ?.
A ? r2
Use the formula for the area of a circle.
22 7
A ?
72
Substitute. Use 7 for r.
7
22 7
49
A ?

Evaluate the power.
1
A ? 22 7
Multiply.
A ? 154
The area of the top that will cover spa is about
154 ft2.
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Additional Example 3 Application
Find the area of the shaded region of the circle.
Use 3.14 for ? . Round your answer to the nearest
tenth.
1.5 cm
The measurement of the radius of the circle is
1.5 cm.
Now find the area of the entire circle.
A ?r2
Use the formula for the area of a circle.

Substitute. Use 1.5 for r.
A 3.14 1.52
A ? 3.14 2.25
Evaluate the power.
A ? 7.07
Multiply.
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Additional Example 3 Continued
Find the area of the shaded region of the circle.
Use 3.14 for ? . Round your answer to the nearest
tenth.
The area of the shaded region of the circle is
about 3.5 cm2.

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Check It Out Example 3
Find the area of the shaded region of the circle.
Use 3.14 for ? . Round your answer to the nearest
tenth.
2.4 cm
The measurement of the radius of the circle is
2.4 cm.
Now find the area of the entire circle.
A ?r2
Use the formula for the area of a circle.

Substitute. Use 2.4 for r.
A 3.14 2.42
A ? 3.14 5.76
Evaluate the power.
A ? 18.09
Multiply.
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Check It Out Example 3 Continued
Find the area of the shaded region of the circle.
Use 3.14 for ? . Round your answer to the nearest
tenth.

The area of the shaded region of the circle is
about 13.56 cm2.