1/2*(142-42) = 77 diagonals
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how many diagonals are there iin a polygon of 11 sides
120*(120-3)/2 = 7020
That polygon is called a "triangle". It has no diagonals.
A polygon with 14 diagonals will have 7 sides
5 diagonals * * * * * That is not correct since two of these would be lines joining the vertex to adjacent vertices (one on either side). These are sides of the polygon, not diagonals. The number of diagonals from any vertex of a polygon with n sides is n-3.