MathDB

Round 6
p16. Let

ABC

be a triangle with

AB = 3

BC = 4

, and

CA = 5

. There exist two possible points

X

CA

such that if

Y

and

Z

are the feet of the perpendiculars from

X

AB

and

BC,

respectively, then the area of triangle

XY Z

1

. If the distance between those two possible points can be expressed as

\frac{a\sqrt{b}}{c}

for positive integers

a

b

, and

c

with

b

squarefree and

gcd(a, c) = 1

, then find

a +b+ c

.
p17. Let

f(n)

be the number of orderings of

1,2, ... ,n

such that each number is as most twice the number preceding it. Find the number of integers

k

between

1

and

50

, inclusive, such that

f (k)

is a perfect square.
p18. Suppose that

f

is a function on the positive integers such that

f(p) = p

for any prime p, and that

f (xy) = f(x) + f(y)

for any positive integers

x

and

y

. Define

g(n) = \sum_{k|n} f (k)

; that is,

g(n)

is the sum of all

f(k)

such that

k

is a factor of

n

. For example,

g(6) = f(1) + 1(2) + f(3) + f(6)

. Find the sum of all composite

n

between

50

and

100

, inclusive, such that

g(n) = n

.
Round 7
p19. AJ is standing in the center of an equilateral triangle with vertices labelled

A

B

, and

C

. They begin by moving to one of the vertices and recording its label; afterwards, each minute, they move to a different vertex and record its label. Suppose that they record

21

labels in total, including the initial one. Find the number of distinct possible ordered triples

(a, b, c)

, where a is the number of

A

's they recorded, b is the number of

B

's they recorded, and c is the number of

C

's they recorded.
p20. Let

S = \sum_{n=1}^{\infty} (1- \{(2 + \sqrt3)^n\})

, where

\{x\} = x - \lfloor x\rfloor

, the fractional part of

x

. If

S =\frac{\sqrt{a} -b}{c}

for positive integers

a, b, c

with

a

squarefree, find

a + b + c

.
p21. Misaka likes coloring. For each square of a

1\times 8

grid, she flips a fair coin and colors in the square if it lands on heads. Afterwards, Misaka places as many

1 \times 2

dominos on the grid as possible such that both parts of each domino lie on uncolored squares and no dominos overlap. Given that the expected number of dominos that she places can be written as

\frac{a}{b}

, for positive integers

a

and

b

with

gcd(a, b) = 1

, find

a + b

.
PS. You should use hide for answers. Rounds 1-3 have been posted [url=https://artofproblemsolving.com/community/c4h3131401p28368159]here and 4-5 [url=https://artofproblemsolving.com/community/c4h3131422p28368457]here. Collected [url=https://artofproblemsolving.com/community/c5h2760506p24143309]here.

Problem Statement