Number of diagonals of a polygon
From each corner draw lines to every corner except itself and its two neighbours. Each diagonal is counted twice.
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How many diagonals does a polygon with 8 sides have?
Answer: 20
How many diagonals does a polygon with 21 sides have?
Answer: 189
How many diagonals does a polygon with 27 sides have?
Answer: 324
Tip: From each corner draw lines to every corner except itself and its two neighbours. Each diagonal is counted twice.
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