1. 11.1 Vocabulary
Circumference
PI ()
Arc Length
Radian

2. The perimeter of a circle is defined as the Circumference.

4. Remember, an arc of a circle has both measure AND length
Remember, an arc of a circle has both measure AND length. You can use the measure of an arc (in degrees) to find it’s length (in linear units). The length of an arc is a fractional amount of the circumference.

7. Degrees and Radians
It is usual to measure angles in degrees. We learn that there are 360 degrees in a circle, that there are 90 degrees in a right angle, and that the angle of an equilateral triangle contains 60 degrees. Another unit of measure for angles and circles is a “radian”
A radian is a measurement of angle, based upon a fundamental property of circles. A radian is the measure of the angle subtended by an arc of a circle whose length is equal to the radius. This turns out to be degrees.
There are 2 radians in a circle (360 degrees).

8. The measure of AB is 1 radian
C = 2  r = 2  radians in a circle
r r