1. Section 7.4: Circumference and Area of a Circle
Postulate 22: The ratio of the circumference of a circle to the length of its diameter is a unique positive constant.
 (pi) is the ratio between the circumference C and the diameter length of any circle; thus  = C/d in any circle.
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Section 7.4 Nack

2. Length of an Arc
Distance between the endpoints of the arc as though it was measured along a straight line.
Theorem 7.4.2: In a circle whose circumference is C, the length l of an arc whose degree measure m is given by: l = m C
360
Example 4 p Figure 7.47
Limits: an approximation of an upper or lower numerical value. See Example 5
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Section 7.4 Nack

3. Ring: A plane figure bounded by concentric circles. Example 8 p. 370
Area of a Circle
Theorem 7.4.3: The area A of a circle whose radius has length r is given by A = r²
Ring: A plane figure bounded by concentric circles.
Example 8 p. 370
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Section 7.4 Nack