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…

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 

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 = 153.9 units2 A = 314.2 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

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