Introduction to Geometry
Definition 1: A location in a plane is called point. It is
denoted by a dot “.” and it has no size and dimensions.
Example 1: A point is located at (4,3)
Definition 4: Ray is a part of a line where it only extends
at one end.
Example 4: A ray formed by points A and B.
Definition 2: A line is a set of all points that extends
infinitely at both ends. In addition, a line can be formed
when two points are connected to each other.
Example 2: Let A and B be two distinct points then line
⃡ is formed.
𝐴𝐵
A
B
Definition 3: Line segment is a part of a line that connects
two points. These two points are called endpoints.
Example 3: Let A and B be two distinct points, then line
segment AB is formed
Definition 5: A formation of two rays with common
endpoints are called angle. The common endpoints are
called vertex.
Example 5: Ray AB and ray AC intersect at point A and
form an angle.
KINDS OF ANGLES
Definition 6: An angle that measures less than 90 degrees
is called Acute Angle.
Definition 7: An angle that measures equal to 90 degrees
is called Right Angle.
Definition 8: An angle that measures greater than 90
degrees is called Obtuse Angle.
Definition 9: An angle that measures equal to 180 degrees
is called Supplementary Angle/Straight Angle.
Definition 10: An angle that measures greater than 180
degrees is called Reflex Angles.
TYPES OF LINES
Definition 11: Two lines that never meets is called Parallel
Lines.
Definition 12: Two lines that intersect at one point is
called Intersecting Lines.
Definition 13: Perpendicular Lines are intersecting lines
that has an angle of 90 degrees or right angle.
Definition 14: Transversal Lines are lines that intersects
a parallel line.
4
5 6
8 7
1
1
1
1
1
1
1
1
1 2
3
1
Observe that <1 and <2 are adjacent angles on the
same line. These angles form a straight line and their sum
is equal to 180 degrees.
Also, observe that <1, <2, <7 and <8 are exterior
angles since it is located outside of the transversal line.
and <3, <4, <5, and <6 are interior angles since it is located
inside of the transversal line. Alternate exterior angles
are exterior angles that lies on the opposite side of the
transversal line while Alternate interior angles are
interior angles that lies on the opposite side of the
transversal line.
Note that <2 and <6 lies on the same side of the
transversal line. These lines are called Corresponding
lines.