Page 157 - Maths Skills - 8

Understanding Quadrilaterals 155

INTRODUCTION
A plane surface is an unobstructed two-dimensional shape like a sheet of paper. If a number of points on a plane
surface are joined without breaking or retracing, the curve so formed is called a plane curve.
Plane curves are of three types:

(i) Open curve (ii) Closed curve (iii) Simple closed curve.

(i) Open curve: A curve which does not intersect itself is an open curve (Fig.).
(ii) Closed curve: A curve which intersects itself is called a closed curve (Fig.).
(iii) Simple closed curve: A closed curve which passes through a given point only once is called a simple
closed curve (Fig.).

POLYGON
A polygon is a simple closed figure made with line segments.

(i) Curves which are polygons (ii) Curves which are not polygons

Convex and Concave Polygons
Convex polygon is a simple polygon, where:

● Each interior angle is less than 180°.
Each line segment between two non-adjacent vertices are in or on the boundary of polygon.

Convex Polygons

Concave polygon is a polygon, that is not convex. It has:
● at least one interior angle greater than 180°.

Concave Polygons