Terms Related to Lines and
Angles
Point
A dot made
with a sharp pencil represents a point. A point has no length or breadth. It
shows a position. Points are represented with capital letters such as A, B, C,
X, Y, Z, P, Q, etc.
Line
A line is an infinite number of points joined together. It may
extend indefinitely in any direction. It is denoted by the symbol ↔. Lines are
named with small letters such as l, m, n, etc.
Types of Lines
Straight Line and Curved Line
Take
a piece of thread. Stretch the thread tightly from the two opposite ends. You
will get a straight line. Now let it loose, you will get a curved line.
Vertical, Horizontal and Oblique Lines
The
standing lines, sleeping lines and slanting lines are called vertical lines,
horizontal lines and oblique lines respectively.
Parallel Lines
The lines m
and n have the same distance between them. If
they are extended indefinitely in both directions and they do not meet, then they are
called parallel lines. We
represent parallel lines m and n as m ‖ n.
Intersecting
Lines
The
lines m and n intersect or cross at O. They are called intersecting lines.
Perpendicular Lines
When
two lines meet or intersect each other at right angles, we say that they are
perpendicular. For example, building or a pole is perpendicular to the ground. We
represent perpendicular lines l and m as l ⊥ m.
Ray
A
line which has one end point and extends endlessly in other direction is called
a ray. Its end point is called initial point. A ray is represented by the
symbol →.
Line Segment
The
straight path between two points is called a line segment. It is the shortest
path between two points.
What is an
Angle?
Take a point O. Draw two
rays OA and OB. The point O is called the vertex, and OA and OB are called the
arms. AOB forms an angle. The space between the arms is called the interior of
the angle, whereas the space outside is called the exterior of the angle. The arms
of an angle are shown with arrow heads.
Naming an
Angle
When
naming an angle the symbol for angle ‘∠’ is written before the
name. The angle is named with the vertex in the middle of the name.
For
example, in the following figure, the angle is named as ∠XYZ or ∠ZYX,
where Y is the vertex, and YX and YZ are the arms.
Interior and Exterior of an Angle
Consider any ∠AOB. It divides
the region of the plane into three parts.
1. The shaded portion of the
plane which is within the two arms of ∠AOB is called
the
interior
of ∠AOB. As the two
rays OA and OB extend endlessly, the interior region also extends endlessly.
2. The portion of the plane
which is outside the two arms of ∠AOB is called
the exterior
of ∠AOB. The exterior
region also extends endlessly.
3. The part of the plane
consisting of all points lying on the arms OA and OB is called the
boundary
of ∠AOB.
4. The interior of ∠AOB along
with its boundary is called the angular
region.
Unit of an Angle
An angle is measured in degree (°).
A degree is subdivided into 60 equal parts and each part is called a minute (‘). A minute can
be divided into 60 equal parts called seconds (“).
Remember that, 1° = 60’ and 1’ = 60”.
Types of Angles
Acute angle
An angle whose measure is
greater than 0° but less than 90°, is called an acute angle.
Right angle
An angle whose measure is
exactly 90°, is called a right angle.
Obtuse angle
An angle whose measure is more
than 90° but less than 180°, is called an obtuse
angle.
Straight angle
An angle whose measure is
exactly 180°, is called a straight angle.
Reflex angle
An angle whose measure is more
than 180° but less than 360°, is called a reflex
angle.
Complete angle
An angle whose measure is 360°, is called a
complete angle.
