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Introduction to Lines and Angles

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Introduction to Lines and Angles - Lesson Summary

Basic Terms and Definitions


A line is a collection of points along a straight path. A line extends in both the directions and has no endpoints. It has no definite length.

Line segment
A line segment is a part of a line with two end points. There is only one line segment joining two given points.

A part of line with one end point is called a ray.

Collinear points
All the points that lie on the same line are called collinear points.

Two rays originating from the same end point form an angle.
Consider the two rays AB and AC originating from the same point A form an angle named as ∠BAC or ∠CAB or ∠A. The rays that form the angle are called the arms of the angle. The end point is called the vertex of the angle. The size of an angle is measured in degrees.

Types of angles

Acute angle

Angles measuring less than 90° but more than 0° are called acute angles.

Right angle
An angle measuring 90° is called a right angle. A right angle is formed by the intersection of two  perpendicular lines.

Obtuse angle
Angles measuring greater than 90° but less than 180° are called obtuse angles.

Straight angle
An angle measuring 180° is called a straight angle.

Reflex angle
Angles measuring greater than 180° but less than 360° are called reflex angles.

Adjacent angles
Two angles are said to be adjacent if they have a common arm and a common vertex.

Linear pair of angles
Two adjacent angles are said to form a linear pair if their sum is 180°. Non-common arms of the linear pair of angles form a straight line

Vertically opposite angles
When two lines intersect, four angles are formed. The angles that are opposite to each other are called vertically opposite angles. There are two pairs of vertically opposite angles.

Complementary angles
Two angles are said to be complementary, if their sum is 90°.

Supplementary angles
Two angles are said to be supplementary if their sum is 180°.

Complementary and supplementary angles may or may not be adjacent angles.

Intersecting lines
Two or more lines that meet at one point are called intersecting lines. When two lines intersect each other, then the vertically opposite angles are equal. Sum of all the angles formed at a point is 360°.

Parallel lines
Lines on the same plane that never intersect are called parallel lines.


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