Lesson 6 of 8
Circles
A circle is all the points the same distance from a center. That distance is the radius. The diameter goes all the way across through the center, so the diameter is twice the radius.
FORMULA
Diameter d = 2r. Circumference (distance around) = 2 pi r, which is the same as pi d. Area = pi r^2. Use pi about 3.14.
Central angles and arcs
- ●A central angle has its vertex at the center of the circle.
- ●An arc is a piece of the circle's edge.
- ●The whole circle is 360 degrees, so a central angle tells you what fraction of the circle the arc covers.
Example: a circle with radius 5. Circumference = 2 times 3.14 times 5 = 31.4. Area = 3.14 times 5^2 = 3.14 times 25 = 78.5. A central angle of 90 degrees marks off one quarter of the circle.