ΒΆ  2016 USAMO Day 1 Problems and Solutions 
Individual Problems and Solutions 
For problems and detailed solutions to each of the 2016 USAMO Day 1  problems, please refer below:
 
Problem 1:  Let X 1 , X 2 , β¦ , X 100  X_{1}, X_{2}, \ldots, X_{100}X 1 β , X 2 β , β¦ , X 1 0 0 β   be a sequence of mutually distinct non-empty subsets of a set S  SS  . Any two sets X i  X_{i}X i β   and X i + 1  X_{i+1}X i + 1 β   are disjoint and their union is not the whole set S  SS  , that is, X i β© X i + 1 = β
  X_{i} \cap X_{i+1}=\emptysetX i β β© X i + 1 β = β
   and X i βͺ X i + 1 β  S  X_{i} \cup X_{i+1} \neq SX i β βͺ X i + 1 β ξ  = S  , for all i β { 1 , β¦ , 99 }  i \in\{1, \ldots, 99\}i β { 1 , β¦ , 9 9 }  . Find the smallest possible number of elements in S  SS  .
Solution: 
View Solution  
Problem 2:  Prove that for any positive integer k  kk  ,
( k 2 ) ! β
 β j = 0 k β 1 j ! ( j + k ) !  \left(k^{2}\right)!\cdot \prod_{j=0}^{k-1} \frac{j!}{(j+k)!}
( k 2 ) ! β
 j = 0 β k β 1 β ( j + k ) ! j ! β 
is an integer.
Solution: 
View Solution  
Problem 3:  Let β³ A B C  \triangle A B Cβ³ A B C   be an acute triangle, and let I B , I C  I_{B}, I_{C}I B β , I C β  , and O  OO   denote its B  BB  -excenter, C  CC  -excenter, and circumcenter, respectively. Points E  EE   and Y  YY   are selected on A C βΎ  \overline{A C}A C   such that β  A B Y = β  C B Y  \angle A B Y=\angle C B Yβ  A B Y = β  C B Y   and B E βΎ β₯ A C βΎ  \overline{B E} \perp \overline{A C}B E β₯ A C  . Similarly, points F  FF   and Z  ZZ   are selected on A B βΎ  \overline{A B}A B   such that β  A C Z = β  B C Z  \angle A C Z=\angle B C Zβ  A C Z = β  B C Z   and C F βΎ β₯ A B βΎ  \overline{C F} \perp \overline{A B}C F β₯ A B  .
Lines I B F β  \overleftrightarrow{I_{B} F}I B β F β   and I C E β  \overleftarrow{I_{C} E}I C β E β   meet at P  PP  . Prove that P O βΎ  \overline{P O}P O   and Y Z βΎ  \overline{Y Z}Y Z   are perpendicular.
Solution: 
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The problems on this page are the property of the MAA's American Mathematics Competitions