Question 191552
polygon  is  "  a  plane  figure  that  meets  two  conditions,  1)  It  is  formed  by  three  or  more  segments  called  sides,  such  that  no  two  sides  with  a  common  endpoint  are  collinear,  and  2)  each  side  intersects  exactly  two  other  sides,  one  at  each  endpoint."    (  Larson,  Geometry)

vertex  is  "  each  endpoint  of  a side  of  a  polygon"    (  Larson,  Geometry)


points  to  watch, 
plane  figure  means  flat  surface ,  2  d  not  3  d,
3  or  more  sides,  triangle  or  better,
angle  between  sides  less  than  180  degrees,
sides  intersect  at  vertex,
also  segment  is  straight  line  not  curved  as  arc