A Diamond is composed of two equilateral triangles set back to back. Fig. [9].)

All other four-sided figures are called Trapeziums. If one opposite pair of sides be parallel, and the other pair not, the figure is called a Trapezoid. Fig. [10].)

Polygons.—A Polygon is a plane rectilineal figure contained by more than four straight lines.

A Regular Polygon is that which has its sides equal, and its angles also are equal.

An Irregular Polygon may have unequal sides and unequal angles, or unequal sides and equal angles, or equal sides and unequal angles. In this chapter regular polygons are only treated of.

Polygons are named according to the number of sides or angles they may have. A polygon having

5sides is a Pentagon.
6“a Hexagon.
7“a Heptagon.
8“an Octagon.
9“a Nonagon.
10“a Decagon.
11“a Undecagon.
12“a Dodecagon.
13“a Tridecagon.
14“a Tetradecagon.
15“a Pentadecagon.
16“a Hexadecagon.
17“a Heptadecagon.
18“an Octadecagon.
19“a Nonodecagon.
20“a Bisdecagon.

Figs. [11], [12], and [13] are self-explanatory.

[Fig. 14] From a given point D without to draw Tangents to a given circle A B C.