
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.
 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.
Terms
Formulas
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 
