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Lines and Angles

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Lines and Angles - Lesson Summary

Length of a line segment:
The distance between the endpoints of a line segment is the length of the line segment. To compare any two line segments, the relation between their lengths is found. Line segments can be compared by observation, by  tracing, by measuring their lengths using a ruler and a divider. The length of a line segment can be measured accurately using a ruler and a divider.

Measurement of angles:
One complete revolution is divided into 360 equal parts. Each part is a degree and written as 360° to say ‘three hundred sixty degrees’. To find the exact measure of an angle, protractor is used.

Types of angles

Complete angle:

An angle of measure 360° is called a complete angle. A comple angle can be divided into four equal parts. Each part is called a quadrant.

Angle formed by one quadrant  =  1 4 (complete angle) = 1 4 x 360° = 90°

Angle formed by two quadrants = 1 2 (complete angle) = 1 2 x 360° = 180°

Angle formed by three quadrants = 3 4 (complete angle) = 3 4 x 360° = 270°

Right angle:
An angle that measures 90° is called a right angle. A right angle makes a quarter revolution.

Straight angle:
An angle that measures 180° is called a straight angle. A straight angle makes a half revolution.

Acute angle:
An angle that measures less than 90° is called an acute angle.

Obtuse angle:
An angle that measures more than 90° and less than 180° is called an obtuse angle.

Reflex angle:
An angle that measures more than 180° is called a reflex angle.

Intersecting lines:
Two lines that meet each other at a single point are called intersecting lines.

Perpendicular lines:
Two lines that intersect each other at right angles are said to be perpendicular to each other.

Bisector of a line segment:
A bisector of a line segment is a line that divides the line segment into two equal parts.

Perpendicular bisector of a line segment:
The perpendicular line that divides a line segment into two equal parts is called the perpendicular bisector of the line segment.

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