2023 AIME II Problem 11
Below is the professionally curated solution for Problem 11 of the 2023 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2023 AIME II solutions, or check the answer key.
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Difficulty rating: 3060
11.
Find the number of collections of distinct subsets of with the property that for any two subsets and in the collection,
Solution:
The subsets split into complementary pairs and no collection can contain both members of a pair (they are disjoint). A collection of pairwise-intersecting subsets must therefore contain exactly one member of every pair; in particular it contains and not
If some singleton is chosen, every member must meet i.e. contain Exactly one set in each complementary pair contains so the collection must be exactly the subsets containing this gives collections. Otherwise no singleton is chosen, so all five -element sets are in the collection. Any two -element subsets of a -element set intersect, a -element set is disjoint only from its complement, and a chosen -element set and a chosen -element set are disjoint only if they are complements, which cannot both be chosen. So the only remaining condition is that the chosen -element sets pairwise intersect.
Viewing -element sets as edges of a pairwise-intersecting collection of edges either has all edges through one common vertex or is a triangle. The number of such edge families is: the empty family (), triangles (), and nonempty families within a star, (subtracting the single edges counted at both endpoints). That is collections, for a total of
Problem 11 in Other Years
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