Page 151 - Math Skill - 5

Geometry 149

Plane Plane

We often come across objects like a sheet of paper Plane
in a notebook, top of a table, the surface of a wall,
etc. These all are smooth flat surfaces. In geometry,
a smooth flat surface that extends endlessly in all Top of a table Sheet of paper
directions is called a plane.
A plane is collection of infinite number of points, lines, line segments and rays.

Fact-o-meter

A plane has length and breadth both and thus it is two-dimensional (2D) whereas
a line, a line segment and a ray are one-dimensional (1D), since they have only
length, no breadth or thickness.

Angles
Take a piece of paper and fold it twice. Now, open the paper.

First fold Second fold

Piece of paper
Now, you will get an angle.

Try to identify angles from your surroundings.
Angles
For example, hands of a clock or the angle between fingers.
Look at this figure.

™ The two rays having a common end point form an A
angle and this common point is called vertex.

™ AC and AB (the two rays forming the angle) are
called the arms of the angle.

The above angle can be written in two ways: C B a
Angle CAB or angle BAC and ∠CAB or ∠BAC. Vertex (here A) is always written in the middle.

Sometimes we also represent an angle by a specific letter or number or some other symbol
like ∠a, ∠CAB, ∠1.

Interior and Exterior of an Angle

We told you in the beginning that when two rays meet, they form an angle. At the same time,
they divide the plane into two parts—one is bounded by the arms of the angle (rays) and the
other lies outside the angle.