Page 218 - ABCTE Study Guide_Neat

Polygons

Lesson Objective

In this lesson, we'll begin to define, identify, and classify polygons.

Previously Covered

So far, we've covered the basics of two-dimensional figures: points, lines, rays, planes, angles, and how
to construct them.

A polygon is a closed figure made by joining line segments, called sides, so that the line segments
intersect exactly two other segments. The point where each pair of segments intersects is called a vertex.

Can you identify the figures below that are polygons and those that are not?

Only figure A is a polygon. Notice that each of the segments that compose the figure is joined with only
two others. Figure B is not a polygon, because it is not made of line segments; figure C is not a polygon,
because it is not closed; and figure D is not a polygon, because two of the segments of which it is
composed intersect three segments.

Triangles

Let’s begin with triangles, since they are the simplest polygons. That is, they are the polygon with the least
number of sides. Then we’ll explore 4-sided polygons, or quadrilaterals, and generalize to polygons with
more than 4 sides.

Triangles may be classified by their sides or by their angles. To do so, remember that two figures are
congruent if they have the same size and the same shape.

There are three classifications of a triangle according to the number of congruent sides it has: scalene,
isosceles, and equilateral.