How many diagonals does a regular undecagon (11-sided polygon) have?
Problem 755
Official solution
Solution:
There are 8 diagonals coming from the first vertex, 8 more from the next, 7 from the next, 6 from the next, 5 from the next, etc., and 1 from the last, for total.
Third method: Each vertex has 8 diagonals touching it. There are 11 vertices. Since each diagonal touches two vertices, this counts every diagonal twice, so there are diagonals.