(i) Polygon  :  It is a closed plane figure bounded by some straight lines.

If   n   =  no.  of   straight  lines,  then  Triangle ( if  n  =  3 ),  Quadrilateral ( if  n  =  4 ),    Pentagon ( if  n  = 5 ),   Hexagon  ( if  n  =  6 ),    Heptagon  ( if  n  =  7 ),  Octagon  ( if  n  =  8 ),  Nonagon ( if  n  =  9 )  and   Decagon ( if  n = 10 ).

  No. of  diagonals of  a  polygon of  n  sides  =  .

 (ii) Concave  Polygon  :   If  at least one interior angle of  a polygon is more than 180⁰, then  it  is  said  to  be  concave  polygon.

 (iii) Convex  Polygon  :   A polygon  in  which  none of  its interior angles is more than  180⁰, is  known  as  a  convex  polygon.

   In  a  convex  polygon  of    ‘n’   sides,  we have  :

▪     Sum of  all interior angles  =  180⁰  ( n – 2 )

▪     Sum of  all exterior angles  =  360⁰

 (iv) Regular  Polygon  :   A  polygon  having  all sides equal and all angles equal is called  a  regular  polygon.

   Each exterior angle of  a regular polygon  = .

   Each interior angle  =  180⁰  –  ( exterior angle ).