What are Polygons?
 A Polygon is a closed figure made up of lines segments (not curves) in twodimensions.
 A minimum of three line segments are required for making a closed figure, thus a polygon with a minimum of three sides is known as Triangle.
Depending on the sides and angles, the polygons are classified into different types, namely
 Regular Polygon
 Irregular Polygon
 Convex Polygon
 Concave polygon
Regular Polygon
If all the sides and interior angles of the polygon are equal, then it is known as a regular polygon.
Irregular Polygon
If all the sides and the interior angles of the polygon are of different measure, then it is known as an irregular polygon.
Convex Polygon
If all the interior angles of a polygon are strictly less than 180 degrees, then it is known as a convex polygon. The vertex will point outwards from the centre of the shape.
Concave Polygon
If one or more interior angles of a polygon are more than 180 degrees, then it is known as a concave polygon. A concave polygon can have at least four sides. The vertex points towards inside of the polygon.
Classification of Triangles
In our article on Geometric Shapes, we discussed the polygons with the least number of sides – the Triangle. In this section, we will go through different types of triangles.
 Equilateral triangle – Having all sides equal.
 Isosceles triangle – Having any 2 sides equal.
 Scalene triangle – All 3 unequal sides.
 Acute angled Triangle– Each angle is less than 90^{0}
 Right Angled Triangle– Any one angle is 90^{0}
 Obtuse Angled Triangle– Any one angle is greater than 90^{0}
An Activity:

An interesting fact about Triangle:

Quadrilaterals
A Quadrilateral is a polygon having the number of sides equal to four.
The table below gives the comparison of Opposite sides, angles, and diagonals of different Quadrilateral.
Quadrilateral  Opposite  Sides  All sides Equal  Opposite angles equal  Diagonal  
Parallel  Equal  Equal  Perpendicular  
Rectangle  ✔  ✔  ✖  ✔  ✔  ✔ 
Parallelogram  ✔  ✔  ✖  ✔  ✖  ✖ 
Rhombus  ✔  ✔  ✔  ✔  ✖  ✔ 
Trapezium  ✖
(Only one side) 
✖  ✖  ✖  ✖  ✖ 
Square  ✔  ✔  ✔  ✔  ✔  ✔ 
Polygon: A general comparison
Polygon  No. of Sides  No. of Diagonal  No. of vertices  Interior Angle 
Triangle  3  0  3  60 
Quadrilateral  4  2  4  90 
Pentagon  5  5  5  108 
Hexagon  6  9  6  120 
Heptagon  7  14  7  128.571 
Octagon  8  20  8  135 
Nonagon  9  27  9  140 
Decagon  10  35  10  144 
Hendecagon  11  44  11  147.273 
Dodecagon  12  54  12  150 
Triskaidecagon  13  65  13  158.308 
Tetrakaidecagon  14  77  14  154.286 
Pentadecagon  15  90  15  156 
Three Dimensional Shape (3D shape)
 These are the shapes that can be projected on a piece of paper but cannot be drawn on a paper. These shapes are known as solids.
2D shape (Polygon)  3D Shape 
Edge  Edge 
Vertices  Vertices 
—  Faces 
 3D shapes have Faces as the distinguishing Feature.
To know more about polygons, types of polygons and 3D shapes, download Byju’s the Learning App.