Mathematics
Grade 7
15 min
Regular and irregular polygons
Regular and irregular polygons
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1
Introduction & Learning Objectives
Learning Objectives
Define what a polygon is and identify its basic components (sides, vertices).
Distinguish between convex and concave polygons.
Identify the key characteristics that define a regular polygon.
Identify the key characteristics that define an irregular polygon.
Classify various polygons as either regular or irregular based on their properties.
Calculate the perimeter of simple regular polygons.
Calculate the perimeter of simple irregular polygons.
Have you ever wondered why a stop sign looks perfectly symmetrical, while a kite might have sides of different lengths? 🤔
In this lesson, we'll dive into the fascinating world of polygons, learning to identify their different types, especially focusing on what makes a polygon 'regular' or 'ir...
2
Key Concepts & Vocabulary
TermDefinitionExample
PolygonA closed two-dimensional shape made up of three or more straight line segments (sides) connected end-to-end.A triangle, a square, a pentagon.
SideOne of the straight line segments that form the boundary of a polygon.In a square, all four line segments are its sides.
Vertex (plural: Vertices)A point where two sides of a polygon meet. It is also called a corner.A triangle has 3 vertices, a square has 4 vertices.
Regular PolygonA polygon where all sides are equal in length AND all interior angles are equal in measure.An equilateral triangle (3 equal sides, 3 equal 60° angles) or a square (4 equal sides, 4 equal 90° angles).
Irregular PolygonA polygon where the sides are not all equal in length, OR the interior angles are not all equal in measure (or both).A recta...
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Core Formulas
Condition for a Regular Polygon
For a polygon to be classified as regular, it must satisfy two conditions simultaneously: 1. All its sides must be equal in length. 2. All its interior angles must be equal in measure.
Use this rule to determine if any given polygon is regular or irregular. If either condition is not met, the polygon is irregular.
Perimeter of a Regular Polygon
$P = n \times s$
To find the perimeter ($P$) of a regular polygon, multiply the number of sides ($n$) by the length of one side ($s$). This works because all sides are equal.
Perimeter of an Irregular Polygon
$P = s_1 + s_2 + ... + s_n$
To find the perimeter ($P$) of an irregular polygon, add the lengths of all its individual sides ($s_1, s_2, ..., s_n$). You must sum each unique side length.
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Challenging
A regular hexagon and a square have the exact same perimeter. If the side length of the regular hexagon is 10 cm, what is the side length of the square?
A.10 cm
B.12 cm
C.15 cm
D.20 cm
Challenging
A regular polygon has a perimeter of 84 cm. Which of the following could NOT be the length of one of its sides?
A.28 cm
B.21 cm
C.12 cm
D.16 cm
Challenging
A certain polygon has at least one interior angle measuring 195°. Based only on this information, what else MUST be true about this polygon?
A.It must have an even number of sides.
B.It must be an irregular polygon.
C.All of its sides must be of different lengths.
D.Its perimeter is greater than 100.
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