Page 184 - ICSE Math 6

17 Triangles and Quadrilaterals

Key Concepts

• Polygon and Types of Polygon • Quadrilateral—Sides, Vertices, Angles, Adjacent
• Triangle—Sides, Vertices, Interior and Exterior, Angles and Opposite Sides, Adjacent and Opposite Angles,
• Median and Altitude of a Triangle Diagonal, Interior and Exterior
• Classification of Triangles • Types of Quadrilaterals

Polygon

A simple closed figure made up of line segments only is called a polygon. In other words, a polygon
is a closed figure formed by straight lines joining only at their end points.

Let’s observe the following closed figures:

(i) (ii) (iii) (iv) (v) (vi)

Figures (i), (ii) and (iv) are examples of polygons. Figures (iii) and (v) are made up of line segments
and curves, hence they are not polygons. Figure (vi) is made up of line segments only, but two of its
sides are intersecting in the middle, hence it is also not a polygon.

Types of polygon

A polygon is said to be regular if all of its sides are equal in length and all of its angles are equal in
measure. A polygon that is not regular is called irregular. Polygons are classified on the basis of the
number of sides. Some of the polygons are given in the following tables.

Polygon Sides Regular Irregular Polygon Sides Regular Irregular

Triangle 3 Hexagon 6

Quadrilateral 4 Heptagon 7

Pentagon 5 Octagon 8

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