SOLUTION: how many diagonals are in a 21-gon?

Question 41213: how many diagonals are in a 21-gon?
Answer by Nate(3500) (Show Source): You can put this solution on YOUR website!
If diagonals are the lines that connect minus the sides to make the 21-gon:
I have thought about polygons like these, and my idea can make this easy:
(1) + (2) + (3) + .... for (s - 1) amount of times where is the number of sides
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ....

minus the sides that connect:

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