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Does the question actually ask how many diagonal does a 4-sided polygon, 5 sided polygon, 6-sided polygon, until 10 have? 

We will try.

4-sided polygon = 2 diagonals
5-sided polygon = 5 diagonals
6-sided polygon = 9 diagonals
7-sided polygon = 14 diagonals
8-sided polygon = 20 diagonals
9-sided polygon = 27 diagonals
10-sided polygon = 35 diagonals
how about diagonal of 3
Do you mean triangle? Actually, if that is the case, the triangle does not have any diagonal. It has only 3 altitudes and not diagonals.