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:
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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:
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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