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Difference between revisions of "2016 UMO Problems"

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*[http://utahmath.org/doc/2016UtahMathOlympiad.pdf 2016 UMO Problems]
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==Problem 1==
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Ada and Otto are engaged in a battle of wits. In front of them is a figure with six dots, and nine sticks are placed between pairs of dots as shown below. The dots are labeled <math>A, B, C, D, E, F</math>. Ada begins the game by placing a pebble on the dot of her choice. Then, starting with Ada and alternating turns, each player picks a stick adjacent to the pebble, moves the pebble to the dot at the other end of the stick, and then removes the stick from the figure. The game ends when there are no sticks adjacent to the pebble. The player who moves last wins. A sample game is described below. If both players play optimally, who will win?
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Sample Game
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1. Ada places the pebble at B.
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2. Ada removes the stick BC, placing the pebble at C.
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3. Otto removes the stick CD, placing the pebble at D.
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4. Ada removes the stick DE, placing the pebble at E.
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5. Otto removes the stick EA, placing the pebble at A.
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6. Ada removes the stick AB and wins.
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[[2016 UMO Problems/Problem 1|Solution]]
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==Problem 2==
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Four fair six-sided dice are rolled. What is the probability that they can be divided into two pairs which sum to the same value? For example, a roll of <math>(1,4,6,3)</math> can be divided into <math>(1,6)</math> and <math>(4,3)</math>, each of which sum to <math>7</math>, but a roll of <math>(1,1,5,2)</math> cannot be divided into two pairs that sum to the same value.
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[[2016 UMO Problems/Problem 2|Solution]]
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==Problem 3==
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Can each positive integer <math>1,2,3,\ldots</math> be colored either red or blue, such that for all positive integers <math>a,b,c,d</math> (not necessarily distinct), if <math>a + b + c = d</math> then <math>a,b,c,d</math> are not all the same color?
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[[2016 UMO Problems/Problem 3|Solution]]
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==Problem 4==
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Equiangular hexagon <math>ABCDEF</math> has <math>AB = CD = EF</math> and <math>AB > BC</math>. Segments <math>AD</math> and <math>CF</math> intersect at point <math>X</math> and segments <math>BE</math> and <math>CF</math> intersect at point <math>Y</math> . If quadrilateral <math>ABYX</math> can have a circle inscribed inside of it (meaning there exists a circle that is tangent to all four sides of the quadrilateral), then find <math>\frac{AB}{FA}</math>.
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[[2016 UMO Problems/Problem 4|Solution]]
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==Problem 5==
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Let <math>a_0,a_1,a_2,\ldots</math> be a sequence of integers (positive, negative, or zero) such that for all nonnegative integers <math>n</math> and <math>k</math>, <math>a_{n+k}^2--2k + 1)a_{n}a_{n+k} + (k^2 + k)a_n^2 = k^2-k</math>. Find all possible sequences (an).
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[[2016 UMO Problems/Problem 5|Solution]]
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==Problem 6==
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Find all positive integer pairs <math>(u,m)</math> such that <math>u + m^2</math> is divisible by <math>um-1</math>.
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[[2016 UMO Problems/Problem 6|Solution]]

Revision as of 04:19, 20 January 2019

Problem 1

Ada and Otto are engaged in a battle of wits. In front of them is a figure with six dots, and nine sticks are placed between pairs of dots as shown below. The dots are labeled $A, B, C, D, E, F$. Ada begins the game by placing a pebble on the dot of her choice. Then, starting with Ada and alternating turns, each player picks a stick adjacent to the pebble, moves the pebble to the dot at the other end of the stick, and then removes the stick from the figure. The game ends when there are no sticks adjacent to the pebble. The player who moves last wins. A sample game is described below. If both players play optimally, who will win?

Sample Game

1. Ada places the pebble at B.

2. Ada removes the stick BC, placing the pebble at C.

3. Otto removes the stick CD, placing the pebble at D.

4. Ada removes the stick DE, placing the pebble at E.

5. Otto removes the stick EA, placing the pebble at A.

6. Ada removes the stick AB and wins.

Solution

Problem 2

Four fair six-sided dice are rolled. What is the probability that they can be divided into two pairs which sum to the same value? For example, a roll of $(1,4,6,3)$ can be divided into $(1,6)$ and $(4,3)$, each of which sum to $7$, but a roll of $(1,1,5,2)$ cannot be divided into two pairs that sum to the same value.


Solution

Problem 3

Can each positive integer $1,2,3,\ldots$ be colored either red or blue, such that for all positive integers $a,b,c,d$ (not necessarily distinct), if $a + b + c = d$ then $a,b,c,d$ are not all the same color?

Solution

Problem 4

Equiangular hexagon $ABCDEF$ has $AB = CD = EF$ and $AB > BC$. Segments $AD$ and $CF$ intersect at point $X$ and segments $BE$ and $CF$ intersect at point $Y$ . If quadrilateral $ABYX$ can have a circle inscribed inside of it (meaning there exists a circle that is tangent to all four sides of the quadrilateral), then find $\frac{AB}{FA}$.

Solution

Problem 5

Let $a_0,a_1,a_2,\ldots$ be a sequence of integers (positive, negative, or zero) such that for all nonnegative integers $n$ and $k$, $a_{n+k}^2--2k + 1)a_{n}a_{n+k} + (k^2 + k)a_n^2 = k^2-k$. Find all possible sequences (an).

Solution

Problem 6

Find all positive integer pairs $(u,m)$ such that $u + m^2$ is divisible by $um-1$.

Solution

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