# 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

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

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**Ray**

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

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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.

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

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**Collinear Points**

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

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Here A, B, C and D are collinear.

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**Angle**

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.Â

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

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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 90Â°. Such an angle is called a right angle. The edges on page, edges of books etc., right angles.

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In the figure given below, ray OA has rotated through half a circle. The angle formed has a measure of 180Â°. 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.

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

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