1. Name of the teacher:ARYA R ARAVIND
Subject : MATHEMATICS
Unit : POLYGONS
Subunit : SUM OF ANGLES OF
A POLYGON
Name of the school :ST . STEPHENS HSS
Standard : VIII
Strength : 38/40
Date : 7/08/2015
Time :40 MIN
PREVIOUS KNOWLEDGE: Students have the knowledge on polygons and sum of angles in a triangle
is 180.
AIDS TO BE USED AS SUPPORT SYSTEM: Polygons having different sides, chart.
Teacher-Pupil activity Phases and their steps
Teacher test previous knowledge of pupil regarding
polygons.State the purpose of the lesson.
ACTIVITY 1
Teacher present a quadrilateral in a paper cut out :
let the pupil observe it. Teacher choose one vertex
in the quadrilateral and skip the next and and
choose the next vertex.Teacher fold it into two
parts. Pupil finds that there are two triangles. And
the sum of angles in a quadrilateral is the sum of
angles in two triangle and is 180+180=2x180=360.
ACTIVITY 2
Teacher shows a pentagon. Teacher first fold it as a
triangle and a quadrilateral.Again teacher fold the
quadrilateral as two triangles. And the sum of
angles is 180+108+180=3x180=540
ACTIVITY3
Teacher present a hexagon and follow the same
procedure above. Pupil finds that the sum of angles
is 720.
Teacher shows heptagon and octagon.
Then teacher tells:
Let us make a table and enter various details in it.
You observe each polygon, find sum of angles in
the polygon and enter values in the column of
table.
Name of the
polygon
Number
of sides
Number
of
triangle
can
make
Sum of
angles
Quadrilateral
Pentagon
Hexagon
Heptagon
Octagon
4
5
6
7
8
2
3
4
5
6
2x180=360
3x180=540
4x180=720
5x180=900
6x180=1080
PHASE 1
CONCEPT FORMATION
TABULATE THE RESULT
2. Make pupil study the result are given above the
table.Then tell them these you find out by really
observing. Now can you give the details without
observing concrete cases.
Teacher asks : What will be the sum of angles in a
nonagon ?
Pupil composes the tabulated result and say 7 x
180 = 1260.
Teacher asks : What will be the sum of angles in a
10 sided polygon ?
Pupil answers that 8 x 180 =1440.
Teacher : Can you give the sum of angles in an n-
sided polygon ?
Pupil : (n - 2)x 180.
Teacher : What can you conclude from this ?
Pupil : The sum of angles in a polygon is the
product of number of triangles can be make in the
polygon and 180.
The sum of angles of an n-sided polygon is
(n - 2)x 180
Give the problem:
(i) What is the sum of angles in 12 sided
polygon ?
Sum of angles = (12-2)x180
= 10 x180
=1800
(ii) Is the sum of angles in any polygon
equal to 2700 ?
(n - 2)x 180 =2700
n-2 =2700/180
=15
n=17
FOLLOW UP ACTIVITIES
Find the sum of the angles of a 62 sided polygon ?
COMPARES AND TRY TO FIND RELATIONS
PROCEEDS TO GENERALIZATION WITH OUT
REALLY OBSERVING.
PHASE 2
INTERPRETING THE DATA
PHASE 3
APPLICATION OF DATA
