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Difference between revisions of "2019 USAJMO Problems"

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===Problem 4===
 
===Problem 4===
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Let <math>ABC</math> be a triangle with <math>\angle ABC</math> obtuse. The <math>A</math>''-excircle'' is a circle in the exterior of <math>\triangle ABC</math> that is tangent to side <math>BC</math> of the triangle and tangent to the extensions of the other two sides. Let <math>E, F</math> be the feet of the altitudes from <math>B</math> and <math>C</math> to lines <math>AC</math> and <math>AB</math>, respectively. Can line <math>EF</math> be tangent to the <math>A</math>-excircle?
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<math>(*)</math> Let <math>ABC</math> be a triangle with <math>\angle ABC</math> obtuse. The <math>A</math>''-excircle'' is a circle in the exterior of <math>\triangle ABC</math> that is tangent to side <math>BC</math> of the triangle and tangent to the extensions of the other two sides. Let <math>E, F</math> be the feet of the altitudes from <math>B</math> and <math>C</math> to lines <math>AC</math> and <math>AB</math>, respectively. Can line <math>EF</math> be tangent to the <math>A</math>-excircle?
  
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[[2019 USAJMO Problems/Problem 4|Solution]]
  
 
===Problem 5===
 
===Problem 5===
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<math>\bullet</math> <math>S_{i,j} \subseteq S_{k,l}</math> whenever <math>0 \le i \le k \le n</math> and <math>0 \le j \le l \le n</math>
 
<math>\bullet</math> <math>S_{i,j} \subseteq S_{k,l}</math> whenever <math>0 \le i \le k \le n</math> and <math>0 \le j \le l \le n</math>
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[[2019 USAJMO Problems/Problem 5|Solution]]
  
 
===Problem 6===
 
===Problem 6===
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Two rational numbers <math>\frac{m}{n}</math> and <math>\frac{n}{m}</math> are written on a blackboard, where <math>m</math> and <math>n</math> are relatively prime positive integers. At any point, Evan may pick two of the numbers <math>x</math> and <math>y</math> written on the board and write either their arithmetic mean <math>\frac{x+y}{2}</math> or their harmonic mean <math>\frac{2xy}{x+y}</math> on the board as well. Find all pairs <math>(m,n)</math> such that Evan can write <math>1</math> on the board in finitely many steps.
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Two rational numbers <math>\tfrac{m}{n}</math> and <math>\tfrac{n}{m}</math> are written on a blackboard, where <math>m</math> and <math>n</math> are relatively prime positive integers. At any point, Evan may pick two of the numbers <math>x</math> and <math>y</math> written on the board and write either their arithmetic mean <math>\tfrac{x+y}{2}</math> or their harmonic mean <math>\tfrac{2xy}{x+y}</math> on the board as well. Find all pairs <math>(m,n)</math> such that Evan can write <math>1</math> on the board in finitely many steps.
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[[2019 USAJMO Problems/Problem 6|Solution]]
  
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{{USAJMO box|year=2019|before=[[2018 USAJMO Problems]]|after=[[2020 USOJMO Problems]]}}
 
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{{USAJMO newbox|year= 2019 |before=[[2018 USAJMO]]|after=[[2020 USAJMO]]}}
 

Latest revision as of 13:21, 10 March 2024

Day 1

Note: For any geometry problem whose statement begins with an asterisk $(*)$, the first page of the solution must be a large, in-scale, clearly labeled diagram. Failure to meet this requirement will result in an automatic 1-point deduction.

Problem 1

There are $a+b$ bowls arranged in a row, numbered $1$ through $a+b$, where $a$ and $b$ are given positive integers. Initially, each of the first $a$ bowls contains an apple, and each of the last $b$ bowls contains a pear.

A legal move consists of moving an apple from bowl $i$ to bowl $i+1$ and a pear from bowl $j$ to bowl $j-1$, provided that the difference $i-j$ is even. We permit multiple fruits in the same bowl at the same time. The goal is to end up with the first $b$ bowls each containing a pear and the last $a$ bowls each containing an apple. Show that this is possible if and only if the product $ab$ is even.

Solution

Problem 2

Let $\mathbb Z$ be the set of all integers. Find all pairs of integers $(a,b)$ for which there exist functions $f:\mathbb Z\rightarrow\mathbb Z$ and $g:\mathbb Z\rightarrow\mathbb Z$ satisfying \[f(g(x))=x+a\quad\text{and}\quad g(f(x))=x+b\] for all integers $x$.

Solution

Problem 3

$(*)$ Let $ABCD$ be a cyclic quadrilateral satisfying $AD^2+BC^2=AB^2$. The diagonals of $ABCD$ intersect at $E$. Let $P$ be a point on side $\overline{AB}$ satisfying $\angle APD=\angle BPC$. Show that line $PE$ bisects $\overline{CD}$.

Solution


Day 2

Problem 4

$(*)$ Let $ABC$ be a triangle with $\angle ABC$ obtuse. The $A$-excircle is a circle in the exterior of $\triangle ABC$ that is tangent to side $BC$ of the triangle and tangent to the extensions of the other two sides. Let $E, F$ be the feet of the altitudes from $B$ and $C$ to lines $AC$ and $AB$, respectively. Can line $EF$ be tangent to the $A$-excircle?

Solution

Problem 5

Let $n$ be a nonnegative integer. Determine the number of ways that one can choose $(n+1)^2$ sets $S_{i,j} \subseteq \{1,2,...,2n\}$, for integers $i,j$ with $0 \le i,j \le n$ such that:

$\bullet$ for all $0 \le i,j \le n$, the set $S_{i,j}$ has $i+j$ elements; and

$\bullet$ $S_{i,j} \subseteq S_{k,l}$ whenever $0 \le i \le k \le n$ and $0 \le j \le l \le n$

Solution

Problem 6

Two rational numbers $\tfrac{m}{n}$ and $\tfrac{n}{m}$ are written on a blackboard, where $m$ and $n$ are relatively prime positive integers. At any point, Evan may pick two of the numbers $x$ and $y$ written on the board and write either their arithmetic mean $\tfrac{x+y}{2}$ or their harmonic mean $\tfrac{2xy}{x+y}$ on the board as well. Find all pairs $(m,n)$ such that Evan can write $1$ on the board in finitely many steps.

Solution

2019 USAJMO (Problems • Resources)
Preceded by
2018 USAJMO Problems 		Followed by
2020 USOJMO Problems
1 • 2 • 3 • 4 • 5 • 6
All USAJMO Problems and Solutions

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