# 2016 UNCO Math Contest II Problems

Twenty-fourth Annual UNC Math Contest Final Round January 23, 2016

Rules: Three hours; no electronic devices. The positive integers are 1, 2, 3, 4, . . .

## Problem 1

Pythagorean Triplet


The sum of the lengths of the three sides of a right triangle is 56. The sum of the squares of the lengths of the three sides of the same right triangle is 1250. What is the area of the triangle?

## Problem 2

Fearsome Foursome


Factorial Find the complete prime factorization of $$\frac{(4!)!}{[(3!)!]^4}$$ (The answer will be a product of powers of eight distinct primes.)

## Problem 3

Polyhedral Die


A cube that is one inch wide has had its eight corners shaved off. The cube’s vertices have been replaced by eight congruent equilateral triangles, and the square faces have been replaced by six congruent octagons. If the combined area of the eight triangles equals the area of one of the octagons, what is that area? (Each octagonal face has two different edge lengths that occur in alternating order.)

## Problem 4

Number Sieve


How many positive integers less than 100 are divisible by exactly two of the numbers 2, 3, 4, 5, 6, 7, 8, 9? For example, 75 is such a number: it is divisible by 3 and by 5, but is not divisible by any of the others on the list. (If you show the integers you find, then you may be assigned partial credit if you have accurately found most of them, even if you do not find all of them.)

## Problem 5

Rock and Roll


Zeus has decreed that Sisyphus must spend each day removing all the rocks in a certain valley and transferring them to Mount Olympus. Each night, each rock Sisyphus places on Mount Olympus is subject to the whims of Zeus: it will either be vaporized (with probability 10%), be rolled back down into the valley (with probability 50% ), or be split by a thunderbolt into two rocks that are both rolled down into the valley (with probability 40%). When the sun rises, Sisyphus returns to work, delivering rocks to Olympus. At sunrise on the first day of his punishment, there is only one rock in the valley and there are no rocks on Mount Olympus. What is the probability that there are exactly two rocks in the valley at sunrise on the third day? (If a rock is vaporized, it is gone.)

## Problem 6

Rock and Roll Forever?


(a) Given the situation in Question $5$, what is the probability that Sisyphus must labor forever? That is, if Sisyphus begins with one rock in the valley on his first morning, what is the probability that the Olympian rocks are never all vaporized? (b) Suppose that the whims of Zeus obey the following rules instead: a rock will either be vaporized (with probability 10%), be rolled back down into the valley (with probability 20%), be split by a thunderbolt into two rocks that are both rolled down into the valley (with probability 30%), or be split by two thunderbolts into three rocks that are all rolled down into the valley (with probability 40%). Now what is the probability that Sisyphus must labor forever?

## Problem 7

Evaluate $$S =\sum_{n=2}^{\infty} \frac{4n}{(n^2-1)^2}$$

## Problem 8

Tree


Each circle in this tree diagram is to be assigned a value, chosen from a set $S$, in such a way that along every pathway down the tree, the assigned values never increase. That is, $A \ge B, A \ge C, C \ge D, C \ge E$, and $A, B, C, D, E \in S$. (It is permissible for a value in $S$ to appear more than once.)

(a) How many ways can the tree be so numbered, using only values chosen from the set $S = \{1, . . . , 6\}$?

(b) Generalize to the case in which $S = \{1, . . . , n\}$. Find a formula for the number of ways the tree can be numbered.

For maximal credit, express your answer in closed form as an explicit algebraic expression in $n$.

## Problem 9

Chess Masters


Four identical white pawns and four identical black pawns are to be placed on a standard 8 × 8, two-colored chessboard.

How many distinct arrangements of the colored pawns on the colored board are possible?

No two pawns occupy the same square. The color of a pawn need not match the color of the square it occupies, but it might. You may give your answer as a formula involving factorials or combinations: you are not asked to compute the number.

## Problem 10

Chess Wallpaper


How many distinct plane wallpaper patterns can be created by cloning the chessboard arrangements described in Question 9?

Each periodic wallpaper pattern is generated by this method: starting with a chessboard arrangement from Question 9 (the master tile), use copies of that tile to fill the plane seamlessly, placing the copies edge-to-edge and corner-to-corner. Note that the resulting wallpaper pattern repeats with period 8, horizontally and vertically. When the tiling is done, the chessboard edges and corners vanish, leaving only an infinite periodic pattern of black and white pawns visible on the wallpaper. Regard two of the infinite wallpaper patterns as the same if and only if there is a plane translation that slides one wallpaper pattern onto an exact copy of the other one. You may slide vertically, horizontally, or a combination of both, any number of squares. (Rotations and reflections are not allowed, just translations.) Note that the wallpaper pattern depicted above can be generated by many different master tiles (by regarding any square 8 × 8 portion of the wallpaper as the master tile chessboard). The challenge is to account for such duplication. Remember that each master tile has exactly four pawns of each color. You may give your answer as an expression using factorials and/or combinations (binomial coefficients). You are not asked to compute the numeric answer.