Terms
Altitude

See Altitude of a Parallelogram, Altitude of a Trapezoid, Altitude of
a Triangle.
Altitude of a Parallelogram

In a parallelogram, the segment with one
endpoint on a side and
perpendicular to that side, with the other
endpoint on the line containing the
opposite side
Altitude of a Trapezoid

In a trapezoid, the segment with one endpoint
on a base and
perpendicular to that base, with the other
endpoint on the line containing the other
base.
Altitude of a Triangle

In a triangle, the segment with one endpoint
on a vertex, and the other endpoint on the
side opposite the vertex, and
perpendicular to that side.
Apothem

A segment with one endpoint at the center
of a regular polygon and the other endpoint at the
midpoint of a
side.
Area

A measurement of the combined length and width of two
dimensional regions.
Base of a Parallelogram

A side containing the endpoint of an altitude.
Center of a Regular Polygon

The point within a regular
polygon that is
equidistant from all
vertices.
Central Angle of a Regular Polygon

An angle created whose
vertex is at the center and whose sides
(rays) extend through the
endpoints of a side.
Circumference

The length of the curve that defines a
circle.
Heron's Formula

A formula that determines the area of a triangle. Named after the mathematician
who first proved the formula worked, Heron's Formula is useful only if you know
the lengths of the sides of a triangle. Heron's Formula states that the area of a
triangle is equal to
, where s is the
semiperimeter of the triangle, and a, b, and c are the lengths of the three
sides.
Perimeter

The length of the simple closed curve or curves
that define a region.
Radius of a Regular Polygon

A segment with one endpoint at the center
and the other endpoint at a vertex of a regular
polygon.
Region

The collection of points that lie within a
simple closed curve.
Semiperimeter

Onehalf of the perimeter.
Square Unit

A square whose sides
are one unit long.
Arc Length

L = (n/360)2(pi)r, where n is the measure of the arc
in degrees, and r is the
radius of the circle.

Area of a Circle

A = (pi * radius)^{2} 
Area of a Circle Segment

A = [(n/360)(pi)(radius)^{2}]  [(1/2)bh], where n is the measure of the
arc in degrees, b
is the measure of the base of the triangle
formed by the radii and the chord, and h is the
length of the altitude of that triangle.

Area of a Parallelogram

A = bh, where b is the length of the base and h is the length of the
altitude.

Area of a Regular Polygon

A = 1/2(ap), where a is the length of the apothem and p is the
perimeter.

Area of a Rhombus

A = 1/2(de), where d and e are the lengths of the
diagonals.

Area of a Sector

A = (n/360)(pi)(radius)^{2}, where n is the measure of the
arc in degrees.

Area of a Square.

A = s^{2}, where s is the length of a side.

Area of a Trapezoid

A = 1/2(h(b_{1} + b_{2}), where h is the length of the
altitude, and b_{1} and b_{2}) are the lengths of the
bases.

Area of a Triangle
  A = 1/2(bh), where b is the length of the base, and h is the length of
the altitude.
 A = Square root [s(sa)(sb)(sc)], where s is the semiperimeter and a,
b, and c are the lengths of the sides of the
triangle.
 A = 1/2((ab)(sin(C)), where a and b are the lengths of two sides and C is
their included angle.
Circumference
C = 2(pi)r, where r is the length of the radius.
