Pi:

The ratio of the circumference

C to diameter

d
of a circle is the constant π (Greek letter pi). The number
π is irrational, and consequantly, a nonrepeating and nonterminating
decimal:

π = 3.1415926535897 ...

Circumference:

The
Circumference:

C
= 2πr
where r =
radius of circle and
π ≈
3.14159...

Diameter:

The diameter of a circle is a straight line through the center of the
circle touching the circle at both sides.

The diameter, d,
of a circle is double its radius:

d = 2r,
where r =
radius of circle

Area
of Circle:

The area enclosed by a circle is the radius squared, multiplied
by π:

A = πr^{2
}

Circular
Sector:

A circular sector or circle sector also known as a
"pie piece" is the portion of a circle enclosed by two radii and an arc.

Let θ be the central angle, in radians,
and *r*
the radius. The total area of a circle is π*r*^{2}.
The area of the sector can be obtained by multiplying the circle's area
by the ratio of the angle and 2π
(because the area of the sector is proportional to the angle, and 2π is the angle for the
whole circle):

A = ½
*r*^{2}θ

In addition, if θ
refers to the central angle in degrees, a similar formula can be
derived.

A
= πr^{2}
· (θ/360)

Circular
Segment:

Let R be the radius of the circle, c the chord length, s the arc length, h the height of the segment, and d portion.
The
the height of the triangulararea of the circular segment is equal to
the area of the circular sector minus the area of the triangular
portion. In the illustration provided below, the circular segment
is represented by the yellow colored portion.
The radius is R = h + d

The arc length is s = Rθ, where θ is in radians