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Names of Polygons

Instructor: Jeremy Cook

I have been teaching elementary school for 16 years. I have extensive experience in lesson and curriculum development and educational technology.

When your mom stops at a stop sign, what shape is it? When you slide into home plate, what shape is the base? Everywhere we look, we can see shapes called polygons. This lesson will teach you what a polygon is and what named we give them.

What are Polygons?

A polygon is a many-sided two-dimensional figure. The word polygon comes from two Greek words. Poly means 'many' and Gon means 'angles.' Many of the names of the polygons also come from Greek words and describe the number of angles. In order to be classified as a polygon, the shape must have three specific traits. It must be a closed figure, meaning that all the sides connect together and leave nothing open. It must be two dimensional, so three-dimensional shapes like cubes and pyramids aren't polygons. Finally, it must have straight sides made of line segments. Any shape with a curved side is not a polygon.

Common Polygons

Let's start with the most basic of polygons, the triangle. Triangles have three enclosed, straight sides but can actually have three names based on their sides; isosceles, equilateral and scalene. They can also have three more names based on their angles; right, obtuse and acute. An equilateral triangle has four equal sides and angles. An isosceles triangle has just two equal sides and angles and a scalene triangle has no equal sides or angles. A right triangle has one right angle, an obtuse triangle has one large angle over 90 degrees and an acute triangle has all small angles that are less than 90 degrees.

Triangles

Polygons with four sides are called quadrilaterals. Within the quadrilateral group, there five different names to remember. A four-sided quadrilateral with four right angles is called a rectangle, and when a rectangle has all four sides of equal length, it's called a square. When a quadrilateral has no right angles it's called a parallelogram, and when that parallelogram has equal length sides it's called a rhombus. Got that yet? Well, there's one more to remember. When the quadrilateral has no right angles and the top and bottom segments are different lengths, it's called a trapezoid. Phew. That's it for four-sided polygons.

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