In geometry, any polygon with ten sides and ten angles and ten vertices is called as Decagon. The sum of angles in a decagon is 1440°.
Decagon Picture

In real life, Decagon shape can be seen in few coins, tiles and mugs. 2 Peso Coin of the Republic of the Philippines has a decagon shape.
The number of diagonals in a polygon is given by the formula $\frac{n(n-3)}{2}$.
Hence, the number of diagonals that can be drawn in a decagon are 35 since n = 10 in a decagon.

A Decagon having all sides of equal length and equal angles is called regular decagon. The figure above shows a regular decagon.
Each of its internal angles is equal to 144° (The formula to find an interior angle of an n-sided polygon is given by $\frac{(n-2) 180}{n}$degrees). In a polygon, the central angle is the angle made at the centre of the polygon by any two adjacent vertices and is given by the formula $\frac{360^o}{n}$. Thus the central angle measure of a regular decagon is 36°.

The number of triangles that can be drawn by drawing the diagonals from a given vertex in general is given by n–2 and so we can draw 8 triangles in a regular decagon.