Bell RingerWednessday
9-5 Area of Circles
AREA OF A CIRCLE
The area A of a
circle is the
product of  and
the square of the
circle’s radius r.
Course 2
A = r2
•
r
9-5 Area of Circles
Additional Example 1A: Finding the Area of a Circle
Find the area of the circle Use 3 for . Know
you have to round up for STAAR
A = r2
7 cm
Use the formula.
A  3 · 72
Substitute 7 for r.
A  3 · 49
Evaluate the power.
Multiply.
A  147
The area of the circle is about 147 cm2.
Remember!
The order of operations calls for evaluating the
exponents before multiplying.
Course 2
9-5 Area of Circles
Check It Out: Example 1A
Find the area of the circle to the nearest tenth.
Use 3 for .
10 cm
Course 2
12 in
2.2 ft
9-5 Area of Circles
Additional Example 1B: Finding the Area of a Circle
Find the area of the circle to the nearest tenth.
Use 3.14 for .
18 ft
A = r2
Use the formula.
R= D/2 r=9
Substitute 9 for r.
Evaluate the power.
Multiply.
A  3 · 81
A  243
The area of the circle is about 243 ft2.
Course 2
9-5 Area of Circles
Check It Out: Example 1B
Use 3 for .
12 ft
Course 2
5 ft
4.8 ft
9-5 Area of Circles
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 3 for .
A =  r2
Use the formula for the area of a circle.
2
A  3 ·14
Substitute. Use 14 for r.
A  3 · 196
Evaluate the power.
A  588
Multiply.
The area of the top that will cover the drain is about 588 in2.
Course 2
9-5 Area of Circles
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?
A radio Station Broadcasts a signal over an
area with a 75-mile radius. What is the area
of the region that Receives the radio signal?
A Circular flower bead in Kay’s backyard has
a diameter of 8 feet. What is the area of the
flower bed?
Course 2
9-5 Area of Circles
Additional Example 3: Application
Find the area of the shaded region of the
circle. Use 3 for  . Round your answer to the
nearest tenth.
The measurement of the radius of the
circle is 1.5 cm.
1.5 cm
Now find the area of the entire circle.
A = r2
Use the formula for the area of a circle.
A = 3 · 1.52
Substitute. Use 1.5 for r.
A  3 · 2.25
Evaluate the power.
A  6.75/2
Course 2
A  3.375 Area of Shaded.