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Problems > High School Math > 2008

## Fractal Function

Let F(x) be a non-decreasing function for all x in [0 1], such that

2F(x/3)=F(x) and F(x)+F(1-x)=1

1. Find F(173/1993) and F(1/13).

2. If possible give a general algorithm to find F(x) for any x.

Other questions:

3. Is F(x) uniquely determined(in [0,1])?

4. Is F(x) continuous(in [0,1])?

Source: unknown

## Card Trick

Charlie has 7 cards numbered 1, 2, 3, 4, 5, 6 and 7 and randomly deals 3 of them to Alice and 3 to Bob. All three people look at the cards that they hold.
Can Alice and Bob communicate with each other, in the presence of Charlie, so that after the communication Alice knows which cards Bob has, and Bob knows which cards Alice has, but, for any card except the one he has, Charlie does not know whether Alice or Bob has it?

Source: unknown

## Reconstruct the Numbers

1. When I sum five numbers in every possible pair combination, I get the values: 0,1,2,4,7,8,9,10,11,12.  What are the original 5 numbers?

2. When I sum a different set of five numbers in every possible group of 3, I get the values: 0,3,4,8,9,10,11,12,14,19.  What are the original 5 numbers?

3. Is it possible to find a set of 5 numbers in either case above which results in the sums 1-10?
Find an example or prove it impossible.

4. If the above problem is not possible, what is the longest series of sequential sums you can find? For example, problems 1 and 2 have six and five sequential sums, (7-12) and (8-12) respectively.

Source: unknown

## Counting Puzzles

1. X is a set with n elements. Find the number of triples (A, B, C), where A, B, C are subsets of X, such that A is a subset of B and B is a subset of C.

2. Let m and n be integers greater than 1. Consider an m*n rectangular
grid of points in the plane. Some k of these points are colored red in such a
way that no three red points are the vertices of a right-angled triangle, two of
whose sides are parallel to the sides of the grid. Determine the greatest
possible value of k for any given values of m,n > 1.

Source: unknown

## A Set of Rational Numbers

Let S be a set of rational numbers with the following properties:

1) 1/2 is an element of S

2) If x is an element of S, then both 1/(x+1) is an element of S and x/(x+1) is an element of S

Prove that S contains all rational numbers in the interval 0

Source: unknown

## Set Sums

Prove that if any set of nine distinct integers has sum greater than 200, then there is a subset of four of the integers whose sum is greater than 100.

Source: unknown

## Trig Is Fun!

Prove the trigonometric identity tan(x - y) + tan(y - z) + tan(z - x) = tan(x - y)
*tan(y -z)*tan(z - x)

Source: unknown

## How Many Solutions?

1. Unique Solution: For which real numbers, a, does the equation

a 3x + 3-x = 3

have a unique solution?

Source: Crux Mathematica, April 2002, from a Finnish high school math contest.

2. Two Solutions: For what values of m does sqrt(x-5)=mx+2 have two solutions?

Source: unknown

## A Mixture Problem

1. A man had a 10-gallon keg of wine and a jug. One day, he drew off a jugful of wine and filled up the keg with water. Later on, when the wine and water had got thoroughly mixed, he drew off another jugful and again filled up the keg with water. The keg then contained equal quantities of wine and water. What was the capacity of the jug?

Source: unknown

## The Power of Imagination

Two problems...

1. What is i to the power i? (Hint: it's a real number!)

2. Give a good approximation* for iii... (Hint: exponentiation is right-associative. That means abc means abc, not (ab)c.)

*for grading purposes, I will define a "good approximation" of complex number x to be a number, z, such that |x-z|<0.01

## Oh, No, Another One of Those ''Year'' Puzzles

1. Prove that 1/64 < (1/2)(3/4)(5/6)... (2009/2010) < 1/44

2. Find the smallest positive integer N such that N! is a multiple of 102009