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

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Question 41213: how many diagonals are in a 21-gon?
Answer by Nate(3500) About Me  (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 s is the number of sides
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 ....
SUM+=+%28n%2F2%29%28a1+%2B+an%29
SUM+=+%2820%2F2%29%281+%2B+20%29
SUM+=+%2810%29%2821%29
SUM+=+210
minus the sides that connect:
210+-+21+=+x
189+=+x