The basic definition of a circle: A round two dimensional plane.

Diameter: The distance across a circle *through its centre.* (usually represented by *d)*

It is NOT this:

It is this:

Radius (Plural is Radii.): The distance from the centre of a circle to its outer edge (usually represented as *r*)

(Some random fact: If you measure from the centre to the outer edge of a "perfect" circle all around, the measurements will be virtually the same. This is what fully defines a circle, a round, two-dimensional plane that measured from the centre to the outer edge, the measurements would be equal in distance.)

We can now safely state that *2r=d*. This means that 2 radii are needed to make the diameter of a circle.

Circumference: The perimeter around the circle, it is usually represented as *C*. (this is what I mean by "outer edge".)

To figure out the circumference, multiply the diameter by Pi (Ï€).

*Pi is approximate to 3.14.*

Some circumference formulas:

C=Ï€d

C=2Ï€r

*(I'll also throw in on how to figure out the area.)*

Some equivalents to a rectangle or parallelogram:

Length/Height: Radius

Width/Base: Half of the circumference

How to figure out the area of a rectangle or parallelogram: Length x Width (or Base x Height)

Enough Parallelogram talk, to figure out the area of a circle is:

*Pi radius square.*