Maths Olympiad Prep

Library / /7 of 97

Geometry Difficulty 7.2 National olympiad, round 2 Find the answer

There are arbitrary 7 points in the plane. Circles are drawn through every 4 possible concyclic points. Find the maximum number of circles that can be drawn.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Given 7 arbitrary points in the plane, we need to determine the maximum number of circles that can be drawn through every 4 possible concyclic points.

To solve this, we consider the combinatorial aspect of selecting 4 points out of 7. The number of ways to choose 4 points from 7 is given by the binomial coefficient:
(74)=7!4!(74)!=76544321=35. \binom{7}{4} = \frac{7!}{4!(7-4)!} = \frac{7 \cdot 6 \cdot 5 \cdot 4}{4 \cdot 3 \cdot 2 \cdot 1} = 35.

However, not all sets of 4 points will necessarily be concyclic. The problem requires us to find the maximum number of circles that can be drawn through any 4 concyclic points. We need to consider the geometric arrangement of points and the possible overlaps of circles.

By considering specific geometric configurations, such as placing the points on the vertices and midpoints of an equilateral triangle, it can be shown that the maximum number of distinct circles that can be drawn through any 4 concyclic points is 7.

Thus, the maximum number of circles that can be drawn through every 4 possible concyclic points among 7 arbitrary points in the plane is:
7. \boxed{7}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.