Circumference of Circles Lesson 9.1.1 Cronnelly

What is circumference? Circumference is a BIG word for the distance around a circle. It is called “perimeter” for other polygons. Cronnelly

How do you calculate the circumference of a circle? You first identify the measurement of the diameter. The diameter is the distance from one side of the circle to the other, crossing through the center of the circle. Once you know the diameter you multiply by pi. Cronnelly

What is Pi? What was the relationship? Mathematicians found a relationship between the diameter of a circle and the circumference of the circle. What was the relationship? 3.141592653589793238462643383279502… A mathematical pattern we now call pi, other wise known as 3.14 Cronnelly

Let’s Check this out!!! Find the circumference: 3.14(5) = 15.7 cm Figure 1 Find the circumference: 3.14(5) = 15.7 cm 5 cm Figure 2 Try this one: 3.14(7) = 21.98 in 7 in Cronnelly

Your Turn!!! Practice 1 Find the circumference: 3.14(30) = 94.2 m 30 m Practice 2 3.14(2.7) = 8.478 ft 2.7 ft Cronnelly

What do we do if we aren’t given the diameter? Sometimes you are given only half of the distance across a circle. This is called the radius of a circle. Cronnelly

What to do with the radius?! You multiply the radius by 2, (because 2 halves make a whole), and then multiply by pi, 3.14! 3(2) = 6 6(3.14) = 18.84 yd 3 yd Cronnelly

Your Turn Now… Purple: 5(2) = 10 3.14(10) = 31.4 km Blue: 3.5(2) = 7 3.5 in 5 km Purple: 5(2) = 10 3.14(10) = 31.4 km Blue: 3.5(2) = 7 3.14(7) = 21.98 in Cronnelly