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Difference between revisions of "2010 USAJMO Problems"

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Let <math>ABC</math> be a triangle with <math>\angle A = 90^{\circ}</math>. Points <math>D</math> and <math>E</math> lie on sides <math>AC</math> and <math>AB</math>, respectively, such that <math>\angle ABD = \angle DBC</math> and <math>\angle ACE = \angle ECB</math>. Segments <math>BD</math> and <math>CE</math> meet at <math>I</math>.  Determine whether or not it is possible for segments <math>AB, AC, BI, ID, CI, IE</math> to all have integer lengths.
 
Let <math>ABC</math> be a triangle with <math>\angle A = 90^{\circ}</math>. Points <math>D</math> and <math>E</math> lie on sides <math>AC</math> and <math>AB</math>, respectively, such that <math>\angle ABD = \angle DBC</math> and <math>\angle ACE = \angle ECB</math>. Segments <math>BD</math> and <math>CE</math> meet at <math>I</math>.  Determine whether or not it is possible for segments <math>AB, AC, BI, ID, CI, IE</math> to all have integer lengths.
  
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[[2010 USAMO Problems/Problem 4|Solution]]
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[[2010 USAJMO Problems/Problem 6|Solution]]
  
 
== See Also ==
 
== See Also ==
 
{{USAJMO box|year=2010|before=First USAJMO|after=[[2011 USAJMO Problems|2011 USAJMO]]}}
 
{{USAJMO box|year=2010|before=First USAJMO|after=[[2011 USAJMO Problems|2011 USAJMO]]}}
 
{{MAA Notice}}
 
{{MAA Notice}}

Latest revision as of 12:55, 16 June 2020

Day 1

Problem 1

A permutation of the set of positive integers $[n] = \{1, 2, \ldots, n\}$ is a sequence $(a_1, a_2, \ldots, a_n)$ such that each element of $[n]$ appears precisely one time as a term of the sequence. For example, $(3, 5, 1, 2, 4)$ is a permutation of $[5]$. Let $P(n)$ be the number of permutations of $[n]$ for which $ka_k$ is a perfect square for all $1\leq k\leq n$. Find with proof the smallest $n$ such that $P(n)$ is a multiple of $2010$.

Solution

Problem 2

Let $n > 1$ be an integer. Find, with proof, all sequences $x_1, x_2, \ldots, x_{n-1}$ of positive integers with the following three properties:

  1. $x_1 < x_2 < \cdots < x_{n-1}$;
  2. $x_i + x_{n-i} = 2n$ for all $i = 1, 2, \ldots, n - 1$;
  3. given any two indices $i$ and $j$ (not necessarily distinct) for which $x_i + x_j < 2n$, there is an index $k$ such that $x_i + x_j = x_k$.

Solution

Problem 3

Let $AXYZB$ be a convex pentagon inscribed in a semicircle of diameter $AB$. Denote by $P, Q, R, S$ the feet of the perpendiculars from $Y$ onto lines $AX, BX, AZ, BZ$, respectively. Prove that the acute angle formed by lines $PQ$ and $RS$ is half the size of $\angle XOZ$, where $O$ is the midpoint of segment $AB$.

Solution

Day 2

Problem 4

A triangle is called a parabolic triangle if its vertices lie on a parabola $y = x^2$. Prove that for every nonnegative integer $n$, there is an odd number $m$ and a parabolic triangle with vertices at three distinct points with integer coordinates with area $(2^nm)^2$.

Solution

Problem 5

Two permutations $a_1, a_2, \ldots, a_{2010}$ and $b_1, b_2, \ldots, b_{2010}$ of the numbers $1, 2, \ldots, 2010$ are said to intersect if $a_k = b_k$ for some value of $k$ in the range $1\leq k\leq 2010$. Show that there exist $1006$ permutations of the numbers $1, 2, \ldots, 2010$ such that any other such permutation is guaranteed to intersect at least one of these $1006$ permutations.

Solution

Problem 6

Let $ABC$ be a triangle with $\angle A = 90^{\circ}$. Points $D$ and $E$ lie on sides $AC$ and $AB$, respectively, such that $\angle ABD = \angle DBC$ and $\angle ACE = \angle ECB$. Segments $BD$ and $CE$ meet at $I$. Determine whether or not it is possible for segments $AB, AC, BI, ID, CI, IE$ to all have integer lengths.

Solution

See Also

2010 USAJMO (Problems • Resources)
Preceded by
First USAJMO 		Followed by
2011 USAJMO
1 • 2 • 3 • 4 • 5 • 6
All USAJMO Problems and Solutions

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