Difference between revisions of "2014 Canadian MO Problems"
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== Problem 1 == | == Problem 1 == | ||
− | Let <math>a_1,a_2,\dots,a_n</math> be positive real numbers whose product is <math>1</math>. Show that the sum <math>\textstyle\frac{a_1}{1+a_1}+\frac{a_2}{(1+a_1)(1+a_2)}+\frac{a_3}{(1+a_1)(1+a_2)(1+a_3)}+\cdots+\frac{a_n}{(1+a_1)(1+a_2)\ | + | Let <math>a_1,a_2,\dots,a_n</math> be positive real numbers whose product is <math>1</math>. Show that the sum <math>\textstyle\frac{a_1}{1+a_1}+\frac{a_2}{(1+a_1)(1+a_2)}+\frac{a_3}{(1+a_1)(1+a_2)(1+a_3)}+\cdots+\frac{a_n}{(1+a_1)(1+a_2)\cdots(1+a_n)}</math> is greater than or equal to <math>\frac{2^n-1}{2^n}</math>. |
− | + | [[2014 Canadian MO Problems/Problem 1|Solution]] | |
== Problem 2== | == Problem 2== | ||
Let <math>m</math> and <math>n</math> be odd positive integers. Each square of an <math>m</math> by <math>n</math> board is coloured red or blue. A row is said to be red-dominated if there are more red squares than blue squares in the row. A column is said to be blue-dominated if there are more blue squares than red squares in the column. Determine the maximum possible value of the number of red-dominated rows plus the number of blue-dominated columns. Express your answer in terms of <math>m</math> and <math>n</math>. | Let <math>m</math> and <math>n</math> be odd positive integers. Each square of an <math>m</math> by <math>n</math> board is coloured red or blue. A row is said to be red-dominated if there are more red squares than blue squares in the row. A column is said to be blue-dominated if there are more blue squares than red squares in the column. Determine the maximum possible value of the number of red-dominated rows plus the number of blue-dominated columns. Express your answer in terms of <math>m</math> and <math>n</math>. | ||
+ | [[2014 Canadian MO Problems/Problem 2|Solution]] | ||
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== Problem 3== | == Problem 3== | ||
Let <math>p</math> be a fixed odd prime. A <math>p</math>-tuple <math>(a_1,a_2,a_3,\ldots,a_p)</math> of integers is said to be good if | Let <math>p</math> be a fixed odd prime. A <math>p</math>-tuple <math>(a_1,a_2,a_3,\ldots,a_p)</math> of integers is said to be good if | ||
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Determine the number of good <math>p</math>-tuples. | Determine the number of good <math>p</math>-tuples. | ||
+ | [[2014 Canadian MO Problems/Problem 3|Solution]] | ||
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== Problem 4== | == Problem 4== | ||
The quadrilateral <math>ABCD</math> is inscribed in a circle. The point <math>P</math> lies in the interior of <math>ABCD</math>, and <math>\angle P AB = \angle P BC = \angle P CD = \angle P DA</math>. The lines <math>AD</math> and <math>BC</math> meet at <math>Q</math>, and the lines <math>AB</math> and <math>CD</math> meet at <math>R</math>. Prove that the lines <math>PQ</math> and <math>PR</math> form the same angle as the diagonals of <math>ABCD</math>. | The quadrilateral <math>ABCD</math> is inscribed in a circle. The point <math>P</math> lies in the interior of <math>ABCD</math>, and <math>\angle P AB = \angle P BC = \angle P CD = \angle P DA</math>. The lines <math>AD</math> and <math>BC</math> meet at <math>Q</math>, and the lines <math>AB</math> and <math>CD</math> meet at <math>R</math>. Prove that the lines <math>PQ</math> and <math>PR</math> form the same angle as the diagonals of <math>ABCD</math>. | ||
+ | [[2014 Canadian MO Problems/Problem 4|Solution]] | ||
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== Problem 5== | == Problem 5== | ||
Fix positive integers <math>n</math> and <math>k\ge 2</math>. A list of n integers is written in a row on a blackboard. You can choose a contiguous block of integers, and I will either add <math>1</math> to all of them or subtract <math>1</math> from all of them. You can repeat this step as often as you like, possibly adapting your selections based on what I do. Prove that after a finite number of steps, you can reach a state where at least <math>n-k+2</math> of the numbers on the blackboard are all simultaneously divisible by <math>k</math>. | Fix positive integers <math>n</math> and <math>k\ge 2</math>. A list of n integers is written in a row on a blackboard. You can choose a contiguous block of integers, and I will either add <math>1</math> to all of them or subtract <math>1</math> from all of them. You can repeat this step as often as you like, possibly adapting your selections based on what I do. Prove that after a finite number of steps, you can reach a state where at least <math>n-k+2</math> of the numbers on the blackboard are all simultaneously divisible by <math>k</math>. | ||
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+ | [[2014 Canadian MO Problems/Problem 5|Solution]] |
Latest revision as of 18:07, 22 January 2017
Problem 1
Let be positive real numbers whose product is . Show that the sum is greater than or equal to .
Problem 2
Let and be odd positive integers. Each square of an by board is coloured red or blue. A row is said to be red-dominated if there are more red squares than blue squares in the row. A column is said to be blue-dominated if there are more blue squares than red squares in the column. Determine the maximum possible value of the number of red-dominated rows plus the number of blue-dominated columns. Express your answer in terms of and .
Problem 3
Let be a fixed odd prime. A -tuple of integers is said to be good if
(i) for all , and (ii) is not divisible by , and (iii) is divisible by .
Determine the number of good -tuples.
Problem 4
The quadrilateral is inscribed in a circle. The point lies in the interior of , and . The lines and meet at , and the lines and meet at . Prove that the lines and form the same angle as the diagonals of .
Problem 5
Fix positive integers and . A list of n integers is written in a row on a blackboard. You can choose a contiguous block of integers, and I will either add to all of them or subtract from all of them. You can repeat this step as often as you like, possibly adapting your selections based on what I do. Prove that after a finite number of steps, you can reach a state where at least of the numbers on the blackboard are all simultaneously divisible by .