A triangle has 3 sides and no diagonals
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That polygon is called a "triangle". It has no diagonals.
There are: 0.5*(1002-(3*100)) = 4850 diagonals
n(n-3)/2
A polygon with n sides had n*(n-3)/2 diagonals. So a 20 sided polygon would have 20*17/3 = 170
Such a polygon does not exist. A polygon with n sides has 0.5*n*(n-3) diagonals If there are 10 diagonals then 0.5*n*(n-3) = 10 which requires n2 - 3n - 20 = 0 which has no integer roots.