This question is similar to, but more difficult than the 2023 Mathcounts Chapter Sprint #30, which is as follows:
Consider the seven points on the circle shown. If George draws line segments connecting pairs of points so that each point is connected to exactly two other points, what is the probability that the resulting figure is a convex heptagon? Express your answer as a common fraction.
Try the question first before you read the solution down below.
Label the points 1 – 7 clockwise around the circle. Each point must be joined to exactly two others, so the drawing is a 2-regular graph (a disjoint union of cycles).
1 . Count all edge–sets (2-regular graphs) on 7 labelled points
- One 7-cycle.
Fix the cyclic order: the edges of a 7-cycle correspond to a permutation of the 6 points after 1, with direction ignored. Hence
\( \dfrac{6!}{2}=360 \) distinct 7-cycles. - One 3-cycle + one 4-cycle.
- Choose the 3-cycle: \( \binom{7}{3}=35 \).
- Orient the 3-cycle: \( (3-1)!/2 = 1 \) way.
- Orient the remaining 4-cycle: \( (4-1)!/2 = 3 \) ways.
Adding the two cases gives the total number of admissible drawings: \[ N_{\text{total}} = 360 + 105 = 465 . \]
2 . Count the favourable edge–set
Exactly one of those drawings is the perimeter \(1\!-\!2\!-\!3\!-\!4\!-\!5\!-\!6\!-\!7\!-\!1\), which yields a convex heptagon.
3 . Probability
\[ \boxed{\displaystyle \Pr(\text{convex heptagon})=\frac{1}{465}} \]Checks: the same counting method gives \(70\) total for 6 points (hexagon) and \(3507\) total for 8 points (octagon), agreeing with \(1/70\) and \(1/3507\) respectively.
