Basics of Geometry

Basics of Geometry




Point: It is an exact location. It is a fine dot which has neither length nor breadth nor thickness but has position i.e., it has no magnitude.

Line segment: The straight path joining two points A and B is called a line segment AB . It has and points and a definite length.

Ray: A line segment which can be extended in only one direction is called a ray.

Intersecting lines: Two lines having a common point are called intersecting lines. The common point is known as the point of intersection.

Concurrent lines: If two or more lines intersect at the same point, then they are known as concurrent lines.

Angles: When two straight lines meet at a point they form an angle.

Right angle: An angle whose measure is 90o is called a right angle.

Acute angle: An angle whose measure is less then one right angle (i.e., less than 90o), is called an acute angle.

Obtuse angle: An angle whose measure is more than one right angle and less than two right angles

(i.e., less than 180o and more than 90o) is called an obtuse angle.

Reflex angle: An angle whose measure is more than 180o and less than 360o is called a reflex angle.

Complementary angles: If the sum of the two angles is one right angle (i.e., 90o), they are called complementary angles. Therefore, the complement of an angle θ is equal to 90° − θ

Supplementary angles: Two angles are said to be supplementary, if the sum of their measures is 180o.  

Example:   Angles measuring 120o and 60o are supplementary angles. Two supplementary angles are the supplement of each other. Therefore, the supplement of an angle θ is equal to 180° − θ.