Definitions
Equilateral triangle.
A triangle all of whose sides have the same length.
Isosceles triangle.
A triangle with two sides of the same length.
Scalene triangle.
A triangle all of whose sides have different lengths.
Right triangle.
A triangle with one right angle.
Obtuse triangle.
A triangle with one obtuse angle.
Right angle.
An angle that measures 90 degrees.
Obtuse angle.
An angle that measures more than 90 degrees.
Acute angle.
An angle that measures less than 90 degrees.
Polygon.
A polygon is a union of line segments which
forms a simple closed curve. This means
that the line segments can intersect only at their endpoints. The endpoints
are called vertices of the polygon, and the line segments
are called the edges or sides of
the polygon. A polygon with a specific number of sides often has a special
name (see triangle, quadrilateral,
pentagon, hexagon, septagon,
octagon, nonagon, decagon,
dodecagon, n-gon.
Also see A
Short Course in Greek and Latin.)
Simple closed curve.
A curve whose beginning is its end, and doesn't intersect itself otherwise.
A simple closed curve in the plane defines an inside (a finite region)
and an outside (an infinite region.)
n-gon.
A polygon with n sides. Usually n
is a specific number; for example, a 17-gon is a 17-sided polygon; a 5-gon
is a pentagon.
Triangle.
A polygon with 3 angles (and so 3 sides).
Quadrilateral.
A polygon with 4 sides.
Pentagon.
A polygon with 5 sides.
Hexagon.
A polygon with 6 sides.
Septagon.
A polygon with 7 sides.
Octagon.
A polygon with 8 sides.
Nonagon.
A polygon with 9 sides.
Decagon.
A polygon with 10 sides.
Dodecagon.
A polygon with 12 sides.
Point.
Undefined term. A point has dimension 0.
Line.
Undefined term. A line has dimension 1.
Plane.
Undefined term. A plane has dimension 2.
3-space.
Undefined term. 3-space has dimension 3.
Line segment.
A line segment with endpoints A and B is the
set of all points on line AB that are between A and B, including A and
B.