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Encapsulate
math
LINES AND ANGLES

Basic Concepts Angle

1. Plane: The geometrical concept of a flat When two rays begin from the P
surface with no edges or thickness is called same point, they form a figure
plane. called an angle. O R
2. Point: A very small dot made with a sharp • The point where both the rays
pencil or a pointed pen on a paper sheet meet is called the vertex.
gives us a point. • Rays that form an angle are called sides or
3. Line: A line is an infinite collection of arms.
points aligned in a straight path.
4. Line segment: A part of a line with two end Angle Bisector
points is called a line segment.
5. Ray: A line segment when extended The line that bisects a given angle into two
endlessly in one direction is called a ray. halves is called the angle bisector of that angle.

Measuring of a Line Segment 20° Angle
20° bisector
To measure a line segment, use a ruler. Align
the ruler so that the zero mark is at one Measuring of an Angle
endpoint and read the measurement at the
other endpoint. For more accurately, we use A protractor is a semicircular
a divider to capture the length. tool used to measure angles.
It is labelled with markings
Perpendicular Lines and Perpendicular from 0° to 180° in both directions (clockwise
Bisector and anti-clockwise).

• If two lines meet at right angles Drawing an Angle
i.e., 90°, they are said to be 90° l
perpendicular. O To draw an angle, create a base ray. Position
• A line that is perpendicular to m the protractor’s centre at the ray’s initial
the line segment and also divides the line point, aligning its zero edge. Mark the desired
segment into two equal parts is called the angle, then use a ruler to connect the points
perpendicular bisector of the line segment. to from an angle.

Types of Angles
Zero angle: 0° Acute angle: 0° < ... < 90° Right angle: 90° Obtuse angle: 90° < ... < 180°

Straight angle: 180° Reflex angle: 180° < ... < 360° Complete angle: 360°

61 Lines and Angles