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Basic Terms and Definitions

Line Segment
A part of a line is called a line segment. It has two end points and its length can be measured. If A and B are the two end points of a line segment, it is named as 'line segment AB ' or



Example : Sides of the polygons, sides of a table etc



A ray is part of a line, which has one end point, and extends indefinitely from the end point.


Its length cannot be measured. If A is the end point and B is any other point on a ray, then the ray can be named as 'ray AB' or .

One way to think of a ray is a line with only one end. A ray starts at a given point and goes off in a certain direction for ever, to infinity. The point where the ray starts is called (confusingly) the endpoint.

 The first letter should denote the end point in a ray. We cannot name the ray shown as .



Collinear Points

If three or more points lie on the same line ,they are collinear points.


Here A, B, C and D are collinear.



An angle is a figure formed by two rays with a common initial point. The common initial point is the vertex and the rays are the arms of the angle. 

In the figure given below, it is in its initial position. The angle formed is 0°.This is called a zero angle.

In the figure given below, it has rotated through a quarter of a circle. If you measure the angle formed you will find that it has a measure of 90°. Such an angle is called a right angle. The edges on page, edges of books etc., right angles.

In the figure given below, ray OA has rotated through half a circle. The angle formed has a measure of 180°. It is called a straight angle. Notice that in this case, the two rays forming the arms of the straight angle are opposite to each other.

In the figure given below, the ray OA has rotated through a full circle. The angle formed is 360°, It is called a complete angle.

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