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Introduction to Lines and Angles

A line is a collection of points along a straight path. A line has no endpoints.
A line segment is a part of a line with two end points.
a part of line with one end point is called a ray.
All the points that lie on the same line are collinear points.
Consider the two rays AB and AC originating from the same point A.
The union of two rays AB and AC is called an angle.
The rays that form the angle are called the arms of the angle.
The intersection point is called the vertex of the angle.
The size of an angle is measured in degrees
An angle that measures less than 90 but more than 0 is called an acute angle.
A right angle is an angle measuring ninety degrees, formed by the intersection of two perpendicular lines.
Angles greater than 90 degrees but less than 180 degrees are known as obtuse angles.
An angle that is equal to 180 degrees is called a straight angle.
A reflex angle is greater than 180 degrees but less than 360 degrees.
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 1800.
Vertically opposite angles: When two lines intersect four angles are formed. The angles that are opposite to each other are called vertically opposite angles.
Two angles are said to be complementary, if their sum is 90 degrees.
Two angles are supplementary if their sum is 180 degrees.
They may or may not be adjacent angles.
Intersecting lines can be defined as two or more lines that meet at one point.
Parallel lines can be defined as lines on the same plane that never intersect.

Theorem: when two lines intersect each other, then the vertically opposite angles are equal.