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Question on Proof of Shoelace Formula

I was looking for a way to prove the shoelace formula when I found this proof: For this clockwise order to make sense, you need a point O inside the polygon so that the angles form $OA_{i}A_{i+1}$ ...
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1answer
398 views

Formulas for finding out if a number is Heptagonal or Octagonal

I am trying to find the formula for finding out if a number is heptagonal. I am also looking for the formula for finding out if a number is octagonal. I already have the formula for finding out the ...