2. Definition of polygons
• A Polygon is any flat shape with three or
more sides. Polygons are closed figures,
which means that the lines are closed and
not open.
6. 2. Concave and convex polygons
• A convex polygon has no internal angle more than
180º and if there are any internal angles greater
than a straight angle, then it is a concave polygon.
8. 3. Regular and Irregular polygons
• Regular polygon is one whose sides are all equal and
whose interior angles are all congruent. Thus, a regular
polygon is both equilateral and equiangular. If, otherwise,
the polygon is said to be irregular polygon.
17. • To construct a polygon’s name, combine the
corresponding prefix and suffix.
Example:
Construct the name for 28 and 46 sided
polygons.
20
Sides Prefix
Icosa
Sides Suffix
8 Octagon
Icosaoctagon
19. Angle – sum property of a polygon
• Proposition 1 : if a polygon has n sides & if all
possible diagonals are drawn from any fixed
vertex, there will be a total of ( n-2) triangles
are formed
• Example : if all possible diagonals are drawn to
a pentagon. Then how many triangles are
formed in pentagon?
Pentagon has 5 sides , n=5
Total no of triangles are formed = 5-2 = 3
20. Proposition -2
• The sum of interior angles of a polygon with n
sides is equal to (2n-4) right angles or (n-2)
straight angles.
• Example : Find the sum of interior angles of
Hexagon.
– Sum of all interior angles of a Hexagon = (n-2) straight angles.
= (6-2) 180
= 4× 180
=720°.
21. Corollary 1
• In a regular polygon with n sides, each interior
angle is equal to
(𝑛−2)
𝑛
180°
• Example : Find the each interior angle of a
pentagon.
– Each interior angle=
(𝑛−2)
𝑛
180
=
(5−2)
5
180
=
3
5
180
= 3× 36
= 108°
22. Proposition 3
• Sum of all its exterior angles = 2n – (2n-4)
right angles = 4 right angles
• Carollary 2 :
• “ in a regular polygons all its exterior angles
are equal to
360
𝑛
angles”.
23. Exercise 4.1.3
• 1. In each of the following ploygons find in
degrees the sum of the interior angles and
the sum of exterior angles.
• (i) Hexagon (ii) Octagon (iii) pentagon
– (iv) Nanogon ( v)