1. A circle is named by the symbol  and its center
A circle is named by the symbol  and its center. A has radius r = AB and diameter d = CD.
Also d = 2r.
Circumference Formula
C = D. Also d = 2r.
Derivation of Area Formula!

2. You can use the circumference of a circle to find its area
You can use the circumference of a circle to find its area. Divide the circle and rearrange the pieces to make a shape that resembles a parallelogram.
The base of the parallelogram is about half the circumference, or r, and the height is close to the radius r. So A   r · r =  r2.
The more pieces you divide the circle into, the more accurate the estimate will be.

4. Example 1A: Finding Measurements of Circles
Find the area of K in terms of .
A = r2
Area of a circle.
Divide the diameter by 2 to find the radius, 3.
A = (3)2
A = 9 in2
Simplify.

5. Example 2: Cooking Application
A pizza-making kit contains three circular baking stones with diameters 24 cm, 36 cm, and 48 cm. Find the area of each stone. Round to the nearest tenth.
24 cm diameter
36 cm diameter
48 cm diameter
A = (12)2
A = (18)2
A = (24)2
≈ cm2
≈ cm2
≈ cm2

6. Check It Out! Example 2
A drum kit contains three drums with diameters of 10 in., 12 in., and 14 in. Find the circumference of each drum.
10 in. diameter in. diameter in. diameter
C = d
C = d
C = d
C = (10)
C = (12)
C = (14)
C = 31.4 in.
C = 37.7 in.
C = 44.0 in.

7. Practice Questions
Find each measurement.
2. the circumference of T in which A = 16 mm2
1. the area of D.
C = 8 mm
A = 49 ft2
3. Speakers come in diameters of 4 in., 9 in., and 16 in. Find the area of each speaker to the nearest tenth.
A1 ≈ 12.6 in2 ; A2 ≈ 63.6 in2 ; A3 ≈ in2