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It totals n(n1)/2. To find that, I just moved the plus and minus terms around (this is allowed because of the commutative property of addition and subtraction). But remember that this includes the sides, not just the diagonals. How many sides does an ngon have? By definition, it has n sides. So we can subtract this from the total. The number of diagonals is n(n1)/2  n. Hope this helps,  


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