Circumference & Surface Area Today’s lesson will cover…  finding circumference and surface area of circles  using formulas to solve problems involving surface area and circumference of circles.

Pi (π) Definition The ratio of circumference to diameter Or Circumference divided by diameter

What we must first know  Definitions  Radius Radius  Diameter Diameter

What we must know first.  Multiply π times 49

What we must know first.  Divide by π

Circumference Definition The distance around the edge of a circle Circumference can be calculated by multiplying the diameter by π If you know the radius, multiply it by 2 to get the diameter

Circumference FORMULAS C = 2πr (use when given radius) Or C = πd (use when given diameter)

Find Circumference given radius  Find the circumference of a circle with a radius of 6 cm.  C = 2πr  C = 2(3.14)(6 cm)  C = 12(3.14)  C = cm

Find Circumference given diameter  Find the circumference of a circle with a diameter of 8 cm.  C = πd  C = (3.14)(8 cm)  C = cm

Area of a Circle  Defined The number of square units it takes to exactly fill the interior of a circle. Interactive circle area Interactive circle areaFormula Area of circle = πr 2

Area Practice  Find the area of the drawn circle. A = πr 2 A = (3.14)(5) 2 A = (3.14)(25) A = 78.5 units 2

Application  A plumber cuts a hole through the subfloor of a new construction so that he can run up a drain pipe to the kitchen sink. The circumference of the hole was 7.85 inches. What is the maximum diameter of pipe that can be run through the floor?

A plumber cuts a hole through the subfloor of a new construction so that he can run up a drain pipe to the kitchen sink. The circumference of the hole was 7.85 inches. What is the maximum diameter of pipe that can be run through the floor?  Given:  Circumference = 7.85 inches  Question: What is the diameter?  Needed: C = πd  Substitute & Solve  7.85cm = 3.14d  D=7.85cm divided by 3.14 = 2.5 cm