Chapter 10Lesson 1: Circles and

Circumference Objective:

Learn to find the circumference of circles.

A circle is the set of all points in a plane that are the same distance from a given point, called the center.

The diameter ( d ) is the distance across a circle through its center.

The circumference ( C ) is the distance around the circle.

The radius ( r ) is the distance from the center to any point on the circle.

Pi ( π )The approximation often used for πis 3.14.

To find the circumference of a circle: To find the circumference of a circle, you have to multiply the diameter ( d ) times Pi ( π ).

Or if they just give you the radius, you must multiply 2 times its radius times Pi.

There are 2 possible formulas:

1) C= π × d or

2) C = 2 × r × π

Example 1:

Find the circumference of the football if it has a diameter of 40 cm.

If the formula is C = π × d, then …

C = 3.14 × 40

C = 125.60cm

Example 2:

Find the circumference.

The formulas are C = π × d or C= 2 × r × πSince the problem gives you the radius we will follow formula #2.

C= 2 × r × π

C = 2 × 28m × 3.14

C = 175.84m

Your Example:

Find the circumference.

Homework:

Find the circumference of each circle.