A circle is something everyone
seems to understand because we see it in everywhere. Simply look at
the full moon, a wheel, or the iris of an eye and you can see the circle
in its natural glory.

The circle has several properties
that make it the starting point of our journey. First of all, it has
a constant relationship (or ratio) of the radius of the circle to the
circumference that is embodied in the constant PI
(pronounced “pie”), which equals around 3.14159. The formula
for the circumference, or distance around the circle, is 2*PI*R, where
R is the radius of the circle. The radius is simply the length of a
straight line drawn from the center of the circle to the outer edge
(see diagram 1). But 2R equals the diameter of the circle (D), which
is a line that cuts the circle into 2 equal pieces, so the circumference
also equals PI * D (see diagram 2).

#### Diagram 1: The Circle Radius

#### Diagram 2: The Circle Diameter

**The Triangle**

The
most basic planar shape, a triangle has the fewest sides (3) needed
to enclose a plane. The primary triangle types are:

- Equilateral
(has all equal angles, at sixty degrees each)

- Isosceles (has
two sides equal length)

- Right (has one
ninety degree angle)