

 Multiplication
 Basic Geometry
First, what is the official definition of a circle?
A circle is a shape that is made up of all the points on a plane (a flat surface) that are the same distance from a given point.
What does that mean?
Okay, so let's start out with the "given point". Let's pick a point on a flat surface, like this screen, for instance. We'll call it point P.
Now pick all the other points on the screen that are the exact same distance R away from point P.
Watch as we add in equal distanced points in the movie below:
Wow, we get a circle!
Terms used in circle geometry
Radius  The radius is the distance from the center point to the edge of the circle. This is the same distance R we used to make the circle in our definition. 
Diameter  The diameter is a straight line that goes across the circle and through the center. It's twice the length of the radius. This gives you our first math equation for the circle: 
Also,
R = D/2
So the diameter is two times the radius and the radius is one half of the diameter.
Circumference  The circumference is the distance around the circle. We use the following formula to figure the circumference:
c = d * π 
Uh oh! What is this symbol π???
This symbol is called pi. It stands for a number that we use with circles. We won't go into the details right now, but let's just agree to use it and that it works for now. Okay?
π = 3.14…..
The decimals actually go on for a long time (forever), but we will round it off to 3.14. It's close enough.
Back to circumference; There are a few handy formulas we will use:
c = d * π
c = 2 * π * r
Where c = circumference, d = diameter, and r = radius.
Other Circle Geometry terms
Sector  A sector is a section of the circle made by two different radii. Sort of like a piece of pizza or pie.
Chord  A chord is a line segment that joins two points on the circle. The diameter is a special type of chord that goes through the center point of the circle. 
More Geometry Subjects
Circle
Polygons
Quadrilaterals
Triangles
Pythagorean Theorem
Perimeter
Slope
Surface Area
Volume of a Box or Cube
Volume and Surface Area of a Sphere
Volume and Surface Area of a Cylinder
Volume and Surface Area of a Cone
Angles glossary
Figures and Shapes glossary
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