Maths Olympiad Prep

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Problem 755

AMC 12 late, AIME early
Combinatorics Difficulty 4.0 Find the answer Annual Harvard-MIT November Tournament · United States

How many diagonals does a regular undecagon (11-sided polygon) have?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

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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 8+8+7+6+5+4+3+2+1=448+8+7+6+5+4+3+2+1=44 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 8112=44\frac{8 \cdot 11}{2}=44 diagonals.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty, ordering) added by this project.