Basic Geometrical Ideas

A point is a mark of position which has neither length nor width or thickness and that does not occupy space. It is usually denoted by a capital letter. A line segment corresponds to the shortest distance between two points. The line segment joining points A and B is denoted by AB with a bar on it. A line segment when extended endlessly on both sides becomes a line. Intersecting lines are two distinct lines which meet at one common point. The lines which never meet each other and lie on the same plane are called parallel lines. A ray is a line that starts from a given fixed point and moves endlessly in one direction. A drawing made without lifting the pencil is known as a curve. A line is also a curve. A simple curve is the one that does not cross itself. A curve that opens in its boundary is called an open curve. A curve with closed boundary is called a closed curve. Two different rays starting from the same fixed point are said to form an angle between them at the starting point. A polygon is a plane closed figure bounded by the line segments. All line segments forming a polygon are known as the sides of the polygon. The meeting point of a pair of sides is called a vertex of the polygon. The end points of the same side of a polygon are known as adjacent vertices. The line obtained by joining two vertices which are not adjacent is called a diagonal. A quadrilateral is a polygon with four sides or edges along with four angles. A simple closed figure which is made up of three line segments is called a triangle. A circle is the set of all those points whose distance from a fixed point remains constant. A line segment with its end-points lying on the circle is called chord of the circle. Diameter is the longest chord in a circle. A line passing through a circle and intersecting the circle at two different points is known as the secant of the circle. The portion (or part) of a circular region enclosed by two radii and the corresponding arc is called the sector of the circle. The portion (or part) of a circular region enclosed between a chord and the corresponding arc is called the segment of the circle. The distance around the circle is the circumference.

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