1. Circles: Circumference & Area
Circle: is the set of all points in a plane that are equidistant from a given point, called the center of the circle. r
Radius: the distance from the center to a point on the circle.
Circumference: the distance around the circle (just like perimeter)
Diameter: distance across the circle, through the center.
Formulas: C = 2r or C = d
: Pi – the ratio of the circles circumference to its diameter  3.14…

2. The radius of a circle is 8, What is the circumference in terms of  ?
Circumference Formulas: C = 2r or C = d  is about 3.14…use symbol in calculator EX
Find the circumference of the following circles. Round your answer to the nearest tenth. 2 10
C = d
C = •10
C = 2r
C = 2••2
C = 4  12.6
C = 10  31.4 EX
The circumference of a circle is 10, What is its diameter in terms of  ?
diameter = EX
The radius of a circle is 8, What is the circumference in terms of  ?
C = 16 

3. Area of a Circle Area = r2 EX
Find the area of the following circle. Round to the nearest tenth. 20 7
A= 72 A= 49 A = units2
A = units squared EX
The area of a circle is given as 95 square units. Find the diameter of the circle, to the nearest whole number.
Diameter = 11 units EX
The area of a circle is given as 25. Find the circumference in terms of .
A = r2 25=r2 25 = r2 5 = r
C = 2r C = 2••r C = 10

4. More Examples: EX
The circumference of a circle is 18. Find the area of the circle, leave answer in terms of .
C = 2  r
A = r2
A = 92
A = 81
18 = 2r
9 = r