MathDB

p1. Given

x + y = 7

, find the value of x that minimizes

4x^2 + 12xy + 9y^2

.
p2. There are real numbers

b

and

c

such that the only

x

-intercept of

8y = x^2 + bx + c

equals its

y

-intercept. Compute

b + c

.

p3. Consider the set of

5

digit numbers

ABCDE

(with

A \ne 0

) such that

A+B = C

B+C = D

, and

C + D = E

. What’s the size of this set?
p4. Let

D

be the midpoint of

BC

\vartriangle ABC

. A line perpendicular to D intersects

AB

E

. If the area of

\vartriangle ABC

is four times that of the area of

\vartriangle BDE

, what is

\angle ACB

in degrees?
p5. Define the sequence

c_0, c_1, ...

with

c_0 = 2

and

c_k = 8c_{k-1} + 5

for

k > 0

. Find

\lim_{k \to \infty} \frac{c_k}{8^k}

.
p6. Find the maximum possible value of

|\sqrt{n^2 + 4n + 5} - \sqrt{n^2 + 2n + 5}|

.
p7. Let

f(x) = \sin^8 (x) + \cos^8(x) + \frac38 \sin^4 (2x)

. Let

f^{(n)}

(x) be the

n

th derivative of

f

. What is the largest integer

a

such that

2^a

divides

f^{(2020)}(15^o)

?
p8. Let

R^n

be the set of vectors

(x_1, x_2, ..., x_n)

where

x_1, x_2,..., x_n

are all real numbers. Let

||(x_1, . . . , x_n)||

denote

\sqrt{x^2_1 +... + x^2_n}

. Let

S

be the set in

R^9

given by

S = \{(x, y, z) : x, y, z \in R^3 , 1 = ||x|| = ||y - x|| = ||z -y||\}.

If a point

(x, y, z)

is uniformly at random from

S

, what is

E[||z||^2]

?
p9. Let

f(x)

be the unique integer between

0

and

x - 1

, inclusive, that is equivalent modulo

x

\left( \sum^2_{i=0} {{x-1} \choose i} ((x - 1 - i)! + i!) \right)

. Let

S

be the set of primes between

3

and

30

, inclusive. Find

\sum_{x\in S}^{f(x)}

.
p10. In the Cartesian plane, consider a box with vertices

(0, 0)

\left( \frac{22}{7}, 0\right)

(0, 24)

\left( \frac{22}{7}, 4\right)

. We pick an integer

a

between

1

and

24

, inclusive, uniformly at random. We shoot a puck from

(0, 0)

in the direction of

\left( \frac{22}{7}, a\right)

and the puck bounces perfectly around the box (angle in equals angle out, no friction) until it hits one of the four vertices of the box. What is the expected number of times it will hit an edge or vertex of the box, including both when it starts at

(0, 0)

and when it ends at some vertex of the box?
p11. Sarah is buying school supplies and she has

\$2019

. She can only buy full packs of each of the following items. A pack of pens is

\$4

, a pack of pencils is

\$3

, and any type of notebook or stapler is

\$1

. Sarah buys at least

1

pack of pencils. She will either buy

1

stapler or no stapler. She will buy at most

3

college-ruled notebooks and at most

2

graph paper notebooks. How many ways can she buy school supplies?
p12. Let

O

be the center of the circumcircle of right triangle

ABC

with

\angle ACB = 90^o

. Let

M

be the midpoint of minor arc

AC

and let

N

be a point on line

BC

such that

MN \perp BC

. Let

P

be the intersection of line

AN

and the Circle

O

and let

Q

be the intersection of line

BP

and

MN

. If

QN = 2

and

BN = 8

, compute the radius of the Circle

O

.
p13. Reduce the following expression to a simplified rational

\frac{1}{1 - \cos \frac{\pi}{9}}+\frac{1}{1 - \cos \frac{5 \pi}{9}}+\frac{1}{1 - \cos \frac{7 \pi}{9}}

p14. Compute the following integral

\int_0^{\infty} \log (1 + e^{-t})dt

.
p15. Define

f(n)

to be the maximum possible least-common-multiple of any sequence of positive integers which sum to

n

. Find the sum of all possible odd

f(n)

PS. You should use hide for answers. Collected [url=https://artofproblemsolving.com/community/c5h2760506p24143309]here.

Problem Statement