olymmath

HMMT 2014

Harvard-MIT Mathematics Tournament 2014.

Algebra

A1

  • Algebra
  • Answer

Given that xx and yy are nonzero real numbers such that x+1y=10x+\frac{1}{y}=10 and y+1x=512y+\frac{1}{x}=\frac{5}{12}, find all possible values of xx.

Answer, solutions and source

A2

  • Algebra
  • Answer

Find the integer closest to

154+14−54−14\frac{1}{\sqrt[4]{5^{4}+1}-\sqrt[4]{5^{4}-1}}

Answer, solutions and source

A3

  • Algebra
  • Answer

Let

A=16((log⁡2(3))3−(log⁡2(6))3−(log⁡2(12))3+(log⁡2(24))3)A=\frac{1}{6}\left(\left(\log _{2}(3)\right)^{3}-\left(\log _{2}(6)\right)^{3}-\left(\log _{2}(12)\right)^{3}+\left(\log _{2}(24)\right)^{3}\right)

Compute 2A2^{A}.

Answer, solutions and source

A4

  • Algebra
  • Answer

Let bb and cc be real numbers, and define the polynomial P(x)=x2+bx+cP(x)=x^{2}+b x+c. Suppose that P(P(1))=P(P(1))= P(P(2))=0P(P(2))=0, and that P(1)≠P(2)P(1) \neq P(2). Find P(0)P(0).

Answer, solutions and source

A5

  • Algebra
  • Answer

Find the sum of all real numbers xx such that 5x4−10x3+10x2−5x−11=05 x^{4}-10 x^{3}+10 x^{2}-5 x-11=0.

Answer, solutions and source

A6

  • Algebra
  • Answer

Given that ww and zz are complex numbers such that ∣w+z∣=1|w+z|=1 and ∣w2+z2∣=14\left|w^{2}+z^{2}\right|=14, find the smallest possible value of ∣w3+z3∣\left|w^{3}+z^{3}\right|. Here, ∣⋅∣|\cdot| denotes the absolute value of a complex number, given by ∣a+bi∣=a2+b2|a+b i|=\sqrt{a^{2}+b^{2}} whenever aa and bb are real numbers.

Answer, solutions and source

A7

  • Algebra
  • Answer

Find the largest real number cc such that

∑i=1101xi2≥cM2\sum_{i=1}^{101} x_{i}^{2} \geq c M^{2}

whenever x1,…,x101x_{1}, \ldots, x_{101} are real numbers such that x1+⋯+x101=0x_{1}+\cdots+x_{101}=0 and MM is the median of x1,…,x101x_{1}, \ldots, x_{101}.

Answer, solutions and source

A8

  • Algebra
  • Answer

Find all real numbers kk such that r4+kr3+r2+4kr+16=0r^{4}+k r^{3}+r^{2}+4 k r+16=0 is true for exactly one real number rr.

Answer, solutions and source

A9

  • Algebra
  • Answer

Given that a,ba, b, and cc are complex numbers satisfying

a2+ab+b2=1+ib2+bc+c2=−2c2+ca+a2=1,\begin{aligned} a^{2}+a b+b^{2} & =1+i \\ b^{2}+b c+c^{2} & =-2 \\ c^{2}+c a+a^{2} & =1, \end{aligned}

compute (ab+bc+ca)2(a b+b c+c a)^{2}. (Here, i=−1i=\sqrt{-1}.)

Answer, solutions and source

A10

  • Algebra
  • Answer

For an integer nn, let f9(n)f_{9}(n) denote the number of positive integers d≤9d \leq 9 dividing nn. Suppose that mm is a positive integer and b1,b2,…,bmb_{1}, b_{2}, \ldots, b_{m} are real numbers such that f9(n)=∑j=1mbjf9(n−j)f_{9}(n)=\sum_{j=1}^{m} b_{j} f_{9}(n-j) for all n>mn>m. Find the smallest possible value of mm.

Answer, solutions and source

Geometry

G1

  • Geometry
  • Answer

Let O1O_{1} and O2O_{2} be concentric circles with radii 4 and 6 , respectively. A chord ABA B is drawn in O1O_{1} with length 2. Extend ABA B to intersect O2O_{2} in points CC and DD. Find CDC D.

Answer, solutions and source

G2

  • Geometry
  • Answer

Point PP and line ℓ\ell are such that the distance from PP to ℓ\ell is 12 . Given that TT is a point on ℓ\ell such that PT=13P T=13, find the radius of the circle passing through PP and tangent to ℓ\ell at TT.

Answer, solutions and source

G3

  • Geometry
  • Answer

ABCA B C is a triangle such that BC=10,CA=12B C=10, C A=12. Let MM be the midpoint of side ACA C. Given that BMB M is parallel to the external bisector of ∠A\angle A, find area of triangle ABCA B C. (Lines ABA B and ACA C form two angles, one of which is ∠BAC\angle B A C. The external bisector of ∠A\angle A is the line that bisects the other angle.)

Answer, solutions and source

G4

  • Geometry
  • Answer

In quadrilateral ABCD,∠DAC=98∘,∠DBC=82∘,∠BCD=70∘A B C D, \angle D A C=98^{\circ}, \angle D B C=82^{\circ}, \angle B C D=70^{\circ}, and BC=ADB C=A D. Find ∠ACD\angle A C D.

Answer, solutions and source

G5

  • Geometry
  • Answer

Let C\mathcal{C} be a circle in the xyx y plane with radius 1 and center (0,0,0)(0,0,0), and let PP be a point in space with coordinates (3,4,8)(3,4,8). Find the largest possible radius of a sphere that is contained entirely in the slanted cone with base C\mathcal{C} and vertex PP.

Answer, solutions and source

G6

  • Geometry
  • Answer

In quadrilateral ABCDA B C D, we have AB=5,BC=6,CD=5,DA=4A B=5, B C=6, C D=5, D A=4, and ∠ABC=90∘\angle A B C=90^{\circ}. Let ACA C and BDB D meet at EE. Compute BEED\frac{B E}{E D}.

Answer, solutions and source

G7

  • Geometry
  • Answer

Triangle ABCA B C has sides AB=14,BC=13A B=14, B C=13, and CA=15C A=15. It is inscribed in circle Γ\Gamma, which has center OO. Let MM be the midpoint of ABA B, let B′B^{\prime} be the point on Γ\Gamma diametrically opposite BB, and let XX be the intersection of AOA O and MB′M B^{\prime}. Find the length of AXA X.

Answer, solutions and source

G8

  • Geometry
  • Answer

Let ABCA B C be a triangle with sides AB=6,BC=10A B=6, B C=10, and CA=8C A=8. Let MM and NN be the midpoints of BAB A and BCB C, respectively. Choose the point YY on ray CMC M so that the circumcircle of triangle AMYA M Y is tangent to ANA N. Find the area of triangle NAYN A Y.

Answer, solutions and source

G9

  • Geometry
  • Answer

Two circles are said to be orthogonal if they intersect in two points, and their tangents at either point of intersection are perpendicular. Two circles ω1\omega_{1} and ω2\omega_{2} with radii 10 and 13 , respectively, are externally tangent at point PP. Another circle ω3\omega_{3} with radius 222 \sqrt{2} passes through PP and is orthogonal to both ω1\omega_{1} and ω2\omega_{2}. A fourth circle ω4\omega_{4}, orthogonal to ω3\omega_{3}, is externally tangent to ω1\omega_{1} and ω2\omega_{2}. Compute the radius of ω4\omega_{4}.

Answer, solutions and source

G10

  • Geometry
  • Answer

Let ABCA B C be a triangle with AB=13,BC=14A B=13, B C=14, and CA=15C A=15. Let Γ\Gamma be the circumcircle of ABCA B C, let OO be its circumcenter, and let MM be the midpoint of minor arc BC^\widehat{B C}. Circle ω1\omega_{1} is internally tangent to Γ\Gamma at AA, and circle ω2\omega_{2}, centered at MM, is externally tangent to ω1\omega_{1} at a point TT. Ray ATA T meets segment BCB C at point SS, such that BS−CS=4/15B S-C S=4 / 15. Find the radius of ω2\omega_{2}.

Answer, solutions and source

Combinatorics

C1

  • Combinatorics
  • Answer

There are 100 students who want to sign up for the class Introduction to Acting. There are three class sections for Introduction to Acting, each of which will fit exactly 20 students. The 100 students, including Alex and Zhu, are put in a lottery, and 60 of them are randomly selected to fill up the classes. What is the probability that Alex and Zhu end up getting into the same section for the class?

Answer, solutions and source

C2

  • Combinatorics
  • Answer

There are 10 people who want to choose a committee of 5 people among them. They do this by first electing a set of 1,2,31,2,3, or 4 committee leaders, who then choose among the remaining people to complete the 5 -person committee. In how many ways can the committee be formed, assuming that people are distinguishable? (Two committees that have the same members but different sets of leaders are considered to be distinct.)

Answer, solutions and source

C3

  • Combinatorics
  • Answer

Bob writes a random string of 5 letters, where each letter is either A,B,CA, B, C, or DD. The letter in each position is independently chosen, and each of the letters A,B,C,DA, B, C, D is chosen with equal probability. Given that there are at least two AA 's in the string, find the probability that there are at least three AA 's in the string.

Answer, solutions and source

C4

  • Combinatorics
  • Answer

Find the number of triples of sets (A,B,C)(A, B, C) such that: (a) A,B,C⊆{1,2,3,…,8}A, B, C \subseteq\{1,2,3, \ldots, 8\}. (b) ∣A∩B∣=∣B∩C∣=∣C∩A∣=2|A \cap B|=|B \cap C|=|C \cap A|=2. (c) ∣A∣=∣B∣=∣C∣=4|A|=|B|=|C|=4.

Here, ∣S∣|S| denotes the number of elements in the set SS.

Answer, solutions and source

C5

  • Combinatorics
  • Answer

Eli, Joy, Paul, and Sam want to form a company; the company will have 16 shares to split among the 4 people. The following constraints are imposed:

  • Every person must get a positive integer number of shares, and all 16 shares must be given out.
  • No one person can have more shares than the other three people combined.

Assuming that shares are indistinguishable, but people are distinguishable, in how many ways can the shares be given out?

Answer, solutions and source

C6

  • Combinatorics
  • Answer

We have a calculator with two buttons that displays an integer xx. Pressing the first button replaces xx by ⌊x2⌋\left\lfloor\frac{x}{2}\right\rfloor, and pressing the second button replaces xx by 4x+14 x+1. Initially, the calculator displays 0 . How many integers less than or equal to 2014 can be achieved through a sequence of arbitrary button presses? (It is permitted for the number displayed to exceed 2014 during the sequence. Here, ⌊y⌋\lfloor y\rfloor denotes the greatest integer less than or equal to the real number yy.)

Answer, solutions and source

C7

  • Combinatorics
  • Answer

Six distinguishable players are participating in a tennis tournament. Each player plays one match of tennis against every other player. There are no ties in this tournament- each tennis match results in a win for one player and a loss for the other. Suppose that whenever AA and BB are players in the tournament such that AA wins strictly more matches than BB over the course of the tournament, it is also true that AA wins the match against BB in the tournament. In how many ways could the tournament have gone?

Answer, solutions and source

C8

  • Combinatorics
  • Answer

The integers 1,2,…,641,2, \ldots, 64 are written in the squares of a 8×88 \times 8 chess board, such that for each 1≤i<641 \leq i<64, the numbers ii and i+1i+1 are in squares that share an edge. What is the largest possible sum that can appear along one of the diagonals?

Answer, solutions and source

C9

  • Combinatorics
  • Answer

There is a heads up coin on every integer of the number line. Lucky is initially standing on the zero point of the number line facing in the positive direction. Lucky performs the following procedure: he looks at the coin (or lack thereof) underneath him, and then,

  • If the coin is heads up, Lucky flips it to tails up, turns around, and steps forward a distance of one unit.
  • If the coin is tails up, Lucky picks up the coin and steps forward a distance of one unit facing the same direction.
  • If there is no coin, Lucky places a coin heads up underneath him and steps forward a distance of one unit facing the same direction.

He repeats this procedure until there are 20 coins anywhere that are tails up. How many times has Lucky performed the procedure when the process stops?

Answer, solutions and source

C10

  • Combinatorics
  • Answer

An up-right path from (a,b)∈R2(a, b) \in \mathbb{R}^{2} to (c,d)∈R2(c, d) \in \mathbb{R}^{2} is a finite sequence (x1,y1),…,(xk,yk)\left(x_{1}, y_{1}\right), \ldots,\left(x_{k}, y_{k}\right) of points in R2\mathbb{R}^{2} such that (a,b)=(x1,y1),(c,d)=(xk,yk)(a, b)=\left(x_{1}, y_{1}\right),(c, d)=\left(x_{k}, y_{k}\right), and for each 1≤i<k1 \leq i<k we have that either (xi+1,yi+1)=(xi+1,yi)\left(x_{i+1}, y_{i+1}\right)=\left(x_{i}+1, y_{i}\right) or (xi+1,yi+1)=(xi,yi+1)\left(x_{i+1}, y_{i+1}\right)=\left(x_{i}, y_{i}+1\right). Two up-right paths are said to intersect if they share any point.

Find the number of pairs (A,B)(A, B) where AA is an up-right path from (0,0)(0,0) to (4,4),B(4,4), B is an up-right path from (2,0)(2,0) to (6,4)(6,4), and AA and BB do not intersect.

Answer, solutions and source

Team

T1

  • Team

Let ω\omega be a circle, and let AA and BB be two points in its interior. Prove that there exists a circle passing through AA and BB that is contained in the interior of ω\omega.

Answer, solutions and source

T2

  • Team
  • Answer

Let a1,a2,…a_{1}, a_{2}, \ldots be an infinite sequence of integers such that aia_{i} divides ai+1a_{i+1} for all i≥1i \geq 1, and let bib_{i} be the remainder when aia_{i} is divided by 210 . What is the maximal number of distinct terms in the sequence b1,b2,…b_{1}, b_{2}, \ldots ?

Answer, solutions and source

T3

  • Team
  • Answer

There are nn girls G1,…,GnG_{1}, \ldots, G_{n} and nn boys B1,…,BnB_{1}, \ldots, B_{n}. A pair (Gi,Bj)\left(G_{i}, B_{j}\right) is called suitable if and only if girl GiG_{i} is willing to marry boy BjB_{j}. Given that there is exactly one way to pair each girl with a distinct boy that she is willing to marry, what is the maximal possible number of suitable pairs?

Answer, solutions and source

T4

  • Team
  • Answer

Compute

∑k=0100⌊2100250+2k⌋\sum_{k=0}^{100}\left\lfloor\frac{2^{100}}{2^{50}+2^{k}}\right\rfloor

(Here, if xx is a real number, then ⌊x⌋\lfloor x\rfloor denotes the largest integer less than or equal to xx.)

Answer, solutions and source

T5

  • Team
  • Answer

Prove that there exists a nonzero complex number cc and a real number dd such that

∥∣11+z+z2∣−∣11+z+z2−c∣∣ =d\left.\|\left|\frac{1}{1+z+z^{2}}\right|-\left|\frac{1}{1+z+z^{2}}-c\right| \right\rvert\,=d

for all zz with ∣z∣=1|z|=1 and 1+z+z2≠01+z+z^{2} \neq 0. (Here, ∣z∣|z| denotes the absolute value of the complex number zz, so that ∣a+bi∣=a2+b2|a+b i|=\sqrt{a^{2}+b^{2}} for real numbers a,ba, b.)

Answer, solutions and source

T6

  • Team

Let nn be a positive integer. A sequence (a0,…,an)\left(a_{0}, \ldots, a_{n}\right) of integers is acceptable if it satisfies the following conditions: (a) 0=∣a0∣<∣a1∣<⋯<∣an−1∣<∣an∣0=\left|a_{0}\right|<\left|a_{1}\right|<\cdots<\left|a_{n-1}\right|<\left|a_{n}\right|. (b) The sets {∣a1−a0∣,∣a2−a1∣,…,∣an−1−an−2∣,∣an−an−1∣}\left\{\left|a_{1}-a_{0}\right|,\left|a_{2}-a_{1}\right|, \ldots,\left|a_{n-1}-a_{n-2}\right|,\left|a_{n}-a_{n-1}\right|\right\} and {1,3,9,…,3n−1}\left\{1,3,9, \ldots, 3^{n-1}\right\} are equal.

Prove that the number of acceptable sequences of integers is (n+1)(n+1) !.

Answer, solutions and source

T8

  • Team
  • Answer

Let ABCA B C be an acute triangle with circumcenter OO such that AB=4,AC=5A B=4, A C=5, and BC=6B C=6. Let DD be the foot of the altitude from AA to BCB C, and EE be the intersection of AOA O with BCB C. Suppose that XX is on BCB C between DD and EE such that there is a point YY on ADA D satisfying XY∥AOX Y \| A O and YO⊥AXY O \perp A X. Determine the length of BXB X.

Answer, solutions and source

T9

  • Team

For integers m,n≥1m, n \geq 1, let A(n,m)A(n, m) be the number of sequences (a1,⋯ ,anm)\left(a_{1}, \cdots, a_{n m}\right) of integers satisfying the following two properties: (a) Each integer kk with 1≤k≤n1 \leq k \leq n occurs exactly mm times in the sequence (a1,⋯ ,anm)\left(a_{1}, \cdots, a_{n m}\right). (b) If i,ji, j, and kk are integers such that 1≤i≤nm1 \leq i \leq n m and 1≤j≤k≤n1 \leq j \leq k \leq n, then jj occurs in the sequence (a1,⋯ ,ai)\left(a_{1}, \cdots, a_{i}\right) at least as many times as kk does.

For example, if n=2n=2 and m=5m=5, a possible sequence is (a1,⋯ ,a10)=(1,1,2,1,2,2,1,2,1,2)\left(a_{1}, \cdots, a_{10}\right)=(1,1,2,1,2,2,1,2,1,2). On the other hand, the sequence (a1,⋯ ,a10)=(1,2,1,2,2,1,1,1,2,2)\left(a_{1}, \cdots, a_{10}\right)=(1,2,1,2,2,1,1,1,2,2) does not satisfy property (2) for i=5,j=1i=5, j=1, and k=2k=2. Prove that A(n,m)=A(m,n)A(n, m)=A(m, n).

Answer, solutions and source

T10

  • Team
  • Answer

Fix a positive real number c>1c>1 and positive integer nn. Initially, a blackboard contains the numbers 1,c,…,cn−11, c, \ldots, c^{n-1}. Every minute, Bob chooses two numbers a,ba, b on the board and replaces them with ca+c2bc a+c^{2} b. Prove that after n−1n-1 minutes, the blackboard contains a single number no less than

(cn/L−1c1/L−1)L\left(\frac{c^{n / L}-1}{c^{1 / L}-1}\right)^{L}

where ϕ=1+52\phi=\frac{1+\sqrt{5}}{2} and L=1+log⁡ϕ(c)L=1+\log _{\phi}(c).

Answer, solutions and source

Guts

U2

  • Guts
  • Answer

Let x1,…,x100x_{1}, \ldots, x_{100} be defined so that for each i,xii, x_{i} is a (uniformly) random integer between 1 and 6 inclusive. Find the expected number of integers in the set {x1,x1+x2,…,x1+x2+…+x100}\left\{x_{1}, x_{1}+x_{2}, \ldots, x_{1}+x_{2}+\ldots+x_{100}\right\} that are multiples of 6 .

Answer, solutions and source

U3

  • Guts
  • Answer

Let ABCDEFA B C D E F be a regular hexagon. Let PP be the circle inscribed in △BDF\triangle B D F. Find the ratio of the area of circle PP to the area of rectangle ABDEA B D E.

Answer, solutions and source

U4

  • Guts
  • Answer

Let DD be the set of divisors of 100 . Let ZZ be the set of integers between 1 and 100, inclusive. Mark chooses an element dd of DD and an element zz of ZZ uniformly at random. What is the probability that dd divides zz ?

Answer, solutions and source

U5

  • Guts
  • Answer

If four fair six-sided dice are rolled, what is the probability that the lowest number appearing on any die is exactly 3 ?

Answer, solutions and source

U7

  • Guts
  • Answer

The Evil League of Evil is plotting to poison the city's water supply. They plan to set out from their headquarters at (5,1)(5,1) and put poison in two pipes, one along the line y=xy=x and one along the line x=7x=7. However, they need to get the job done quickly before Captain Hammer catches them. What's the shortest distance they can travel to visit both pipes and then return to their headquarters?

Answer, solutions and source

U8

  • Guts
  • Answer

The numbers 20,21,⋯ ,215,216=655362^{0}, 2^{1}, \cdots, 2^{15}, 2^{16}=65536 are written on a blackboard. You repeatedly take two numbers on the blackboard, subtract one from the other, erase them both, and write the result of the subtraction on the blackboard. What is the largest possible number that can remain on the blackboard when there is only one number left?

Answer, solutions and source

U9

  • Guts
  • Answer

Compute the side length of the largest cube contained in the region

{(x,y,z):x2+y2+z2≤25 and x≥0}\left\{(x, y, z): x^{2}+y^{2}+z^{2} \leq 25 \text { and } x \geq 0\right\}

of three-dimensional space.

Answer, solutions and source

U10

  • Guts
  • Answer

Find the number of nonempty sets F\mathcal{F} of subsets of the set {1,…,2014}\{1, \ldots, 2014\} such that: (a) For any subsets S1,S2∈F,S1∩S2∈FS_{1}, S_{2} \in \mathcal{F}, S_{1} \cap S_{2} \in \mathcal{F}. (b) If S∈F,T⊆{1,…,2014}S \in \mathcal{F}, T \subseteq\{1, \ldots, 2014\}, and S⊆TS \subseteq T, then T∈FT \in \mathcal{F}.

Answer, solutions and source

U11

  • Guts
  • Answer

Two fair octahedral dice, each with the numbers 1 through 8 on their faces, are rolled. Let NN be the remainder when the product of the numbers showing on the two dice is divided by 8 . Find the expected value of NN.

Answer, solutions and source

U12

  • Guts
  • Answer

Find a nonzero monic polynomial P(x)P(x) with integer coefficients and minimal degree such that P(1−23+43)=0P(1-\sqrt[3]{2}+\sqrt[3]{4})=0. (A polynomial is called monic if its leading coefficient is 1.)

Answer, solutions and source

U13

  • Guts
  • Answer

An auditorium has two rows of seats, with 50 seats in each row. 100 indistinguishable people sit in the seats one at a time, subject to the condition that each person, except for the first person to sit in each row, must sit to the left or right of an occupied seat, and no two people can sit in the same seat. In how many ways can this process occur?

Answer, solutions and source

U14

  • Guts
  • Answer

Let ABCDA B C D be a trapezoid with AB∥CDA B \| C D and ∠D=90∘\angle D=90^{\circ}. Suppose that there is a point EE on CDC D such that AE=BEA E=B E and that triangles AEDA E D and CEBC E B are similar, but not congruent. Given that CDAB=2014\frac{C D}{A B}=2014, find BCAD\frac{B C}{A D}.

Answer, solutions and source

U15

  • Guts
  • Answer

Given a regular pentagon of area 1, a pivot line is a line not passing through any of the pentagon's vertices such that there are 3 vertices of the pentagon on one side of the line and 2 on the other. A pivot point is a point inside the pentagon with only finitely many non-pivot lines passing through it. Find the area of the region of pivot points.

Answer, solutions and source

U16

  • Guts
  • Answer

Suppose that xx and yy are positive real numbers such that x2−xy+2y2=8x^{2}-x y+2 y^{2}=8. Find the maximum possible value of x2+xy+2y2x^{2}+x y+2 y^{2}.

Answer, solutions and source

U17

  • Guts
  • Answer

Let f:N→Nf: \mathbb{N} \rightarrow \mathbb{N} be a function satisfying the following conditions: (a) f(1)=1f(1)=1. (b) f(a)≤f(b)f(a) \leq f(b) whenever aa and bb are positive integers with a≤ba \leq b. (c) f(2a)=f(a)+1f(2 a)=f(a)+1 for all positive integers aa.

How many possible values can the 2014-tuple (f(1),f(2),…,f(2014))(f(1), f(2), \ldots, f(2014)) take?

Answer, solutions and source

U18

  • Guts
  • Answer

Find the number of ordered quadruples of positive integers (a,b,c,d)(a, b, c, d) such that a,b,ca, b, c, and dd are all (not necessarily distinct) factors of 30 and abcd>900a b c d>900.

Answer, solutions and source

U19

  • Guts
  • Answer

Let ABCDA B C D be a trapezoid with AB∥CDA B \| C D. The bisectors of ∠CDA\angle C D A and ∠DAB\angle D A B meet at EE, the bisectors of ∠ABC\angle A B C and ∠BCD\angle B C D meet at FF, the bisectors of ∠BCD\angle B C D and ∠CDA\angle C D A meet at GG, and the bisectors of ∠DAB\angle D A B and ∠ABC\angle A B C meet at HH. Quadrilaterals EABFE A B F and EDCFE D C F have areas 24 and 36, respectively, and triangle ABHA B H has area 25 . Find the area of triangle CDGC D G.

Answer, solutions and source

U20

  • Guts
  • Answer

A deck of 8056 cards has 2014 ranks numbered 1-2014. Each rank has four suits-hearts, diamonds, clubs, and spades. Each card has a rank and a suit, and no two cards have the same rank and the same suit. How many subsets of the set of cards in this deck have cards from an odd number of distinct ranks?

Answer, solutions and source

U21

  • Guts
  • Answer

Compute the number of ordered quintuples of nonnegative integers ( a1,a2,a3,a4,a5a_{1}, a_{2}, a_{3}, a_{4}, a_{5} ) such that 0≤a1,a2,a3,a4,a5≤70 \leq a_{1}, a_{2}, a_{3}, a_{4}, a_{5} \leq 7 and 5 divides 2a1+2a2+2a3+2a4+2a52^{a_{1}}+2^{a_{2}}+2^{a_{3}}+2^{a_{4}}+2^{a_{5}}.

Answer, solutions and source

U22

  • Guts
  • Answer

Let ω\omega be a circle, and let ABCDA B C D be a quadrilateral inscribed in ω\omega. Suppose that BDB D and ACA C intersect at a point EE. The tangent to ω\omega at BB meets line ACA C at a point FF, so that CC lies between EE and FF. Given that AE=6,EC=4,BE=2A E=6, E C=4, B E=2, and BF=12B F=12, find DAD A.

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U23

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Let S={−100,−99,−98,…,99,100}S=\{-100,-99,-98, \ldots, 99,100\}. Choose a 50 -element subset TT of SS at random. Find the expected number of elements of the set {∣x∣:x∈T}\{|x|: x \in T\}.

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U24

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Let A={a1,a2,…,a7}A=\left\{a_{1}, a_{2}, \ldots, a_{7}\right\} be a set of distinct positive integers such that the mean of the elements of any nonempty subset of AA is an integer. Find the smallest possible value of the sum of the elements in AA.

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U25

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Let ABCA B C be an equilateral triangle of side length 6 inscribed in a circle ω\omega. Let A1,A2A_{1}, A_{2} be the points (distinct from AA ) where the lines through AA passing through the two trisection points of BCB C meet ω\omega. Define B1,B2,C1,C2B_{1}, B_{2}, C_{1}, C_{2} similarly. Given that A1,A2,B1,B2,C1,C2A_{1}, A_{2}, B_{1}, B_{2}, C_{1}, C_{2} appear on ω\omega in that order, find the area of hexagon A1A2B1B2C1C2A_{1} A_{2} B_{1} B_{2} C_{1} C_{2}.

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U26

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For 1≤j≤20141 \leq j \leq 2014, define

bj=j2014∏i=1,i≠j2014(i2014−j2014)b_{j}=j^{2014} \prod_{i=1, i \neq j}^{2014}\left(i^{2014}-j^{2014}\right)

where the product is over all i∈{1,…,2014}i \in\{1, \ldots, 2014\} except i=ji=j. Evaluate

1b1+1b2+⋯+1b2014\frac{1}{b_{1}}+\frac{1}{b_{2}}+\cdots+\frac{1}{b_{2014}}

Answer, solutions and source

U27

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Suppose that (a1,…,a20)\left(a_{1}, \ldots, a_{20}\right) and (b1,…,b20)\left(b_{1}, \ldots, b_{20}\right) are two sequences of integers such that the sequence (a1,…,a20,b1,…,b20)\left(a_{1}, \ldots, a_{20}, b_{1}, \ldots, b_{20}\right) contains each of the numbers 1,…,401, \ldots, 40 exactly once. What is the maximum possible value of the sum

∑i=120∑j=120min⁡(ai,bj)?\sum_{i=1}^{20} \sum_{j=1}^{20} \min \left(a_{i}, b_{j}\right) ?

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U28

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Let f(n)f(n) and g(n)g(n) be polynomials of degree 2014 such that f(n)+(−1)ng(n)=2nf(n)+(-1)^{n} g(n)=2^{n} for n=n= 1,2,…,40301,2, \ldots, 4030. Find the coefficient of x2014x^{2014} in g(x)g(x).

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U29

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Natalie has a copy of the unit interval [0,1][0,1] that is colored white. She also has a black marker, and she colors the interval in the following manner: at each step, she selects a value x∈[0,1]x \in[0,1] uniformly at random, and (a) If x≤12x \leq \frac{1}{2} she colors the interval [x,x+12]\left[x, x+\frac{1}{2}\right] with her marker. (b) If x>12x>\frac{1}{2} she colors the intervals [x,1][x, 1] and [0,x−12]\left[0, x-\frac{1}{2}\right] with her marker.

What is the expected value of the number of steps Natalie will need to color the entire interval black?

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U30

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Let ABCA B C be a triangle with circumcenter OO, incenter I,∠B=45∘I, \angle B=45^{\circ}, and OI∥BCO I \| B C. Find cos⁡∠C\cos \angle C.

Answer, solutions and source

U32

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Find all ordered pairs (a,b)(a, b) of complex numbers with a2+b2≠0,a+10ba2+b2=5a^{2}+b^{2} \neq 0, a+\frac{10 b}{a^{2}+b^{2}}=5, and b+10aa2+b2=4b+\frac{10 a}{a^{2}+b^{2}}=4.

Answer, solutions and source

U33

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An up-right path from (a,b)∈R2(a, b) \in \mathbb{R}^{2} to (c,d)∈R2(c, d) \in \mathbb{R}^{2} is a finite sequence (x1,y1),…,(xk,yk)\left(x_{1}, y_{1}\right), \ldots,\left(x_{k}, y_{k}\right) of points in R2\mathbb{R}^{2} such that (a,b)=(x1,y1),(c,d)=(xk,yk)(a, b)=\left(x_{1}, y_{1}\right),(c, d)=\left(x_{k}, y_{k}\right), and for each 1≤i<k1 \leq i<k we have that either (xi+1,yi+1)=(xi+1,yi)\left(x_{i+1}, y_{i+1}\right)=\left(x_{i}+1, y_{i}\right) or (xi+1,yi+1)=(xi,yi+1)\left(x_{i+1}, y_{i+1}\right)=\left(x_{i}, y_{i}+1\right). Let SS be the set of all up-right paths from (−400,−400)(-400,-400) to (400,400)(400,400). What fraction of the paths in SS do not contain any point (x,y)(x, y) such that ∣x∣,∣y∣≤10|x|,|y| \leq 10 ? Express your answer as a decimal number between 0 and 1. If CC is the actual answer to this question and AA is your answer, then your score on this problem is ⌈max⁡{25(1−10∣C−A∣),0}⌉\lceil\max \{25(1-10|C-A|), 0\}\rceil.

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U34

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Consider a number line, with a lily pad placed at each integer point. A frog is standing at the lily pad at the point 0 on the number line, and wants to reach the lily pad at the point 2014 on the number line. If the frog stands at the point nn on the number line, it can jump directly to either point n+2n+2 or point n+3n+3 on the number line. Each of the lily pads at the points 1,⋯ ,20131, \cdots, 2013 on the number line has, independently and with probability 1/21 / 2, a snake. Let pp be the probability that the frog can make some sequence of jumps to reach the lily pad at the point 2014 on the number line, without ever landing on a lily pad containing a snake. What is p1/2014p^{1 / 2014} ? Express your answer as a decimal number. If CC is the actual answer to this question and AA is your answer, then your score on this problem is ⌈max⁡{25(1−20∣C−A∣),0}⌉\lceil\max \{25(1-20|C-A|), 0\}\rceil.

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U35

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How many times does the letter "e" occur in all problem statements in this year's HMMT February competition? If CC is the actual answer to this question and AA is your answer, then your score on this problem is ⌈max⁡{25(1−∣log⁡2(C/A)∣),0}⌉\left\lceil\max \left\{25\left(1-\left|\log _{2}(C / A)\right|\right), 0\right\}\right\rceil.

Answer, solutions and source

U36

  • Guts

We have two concentric circles C1C_{1} and C2C_{2} with radii 1 and 2 , respectively. A random chord of C2C_{2} is chosen. What is the probability that it intersects C1C_{1} ? Your answer to this problem must be expressed in the form mn\frac{m}{n}, where mm and nn are positive integers. If your answer is in this form, your score for this problem will be ⌊25⋅XY⌋\left\lfloor\frac{25 \cdot X}{Y}\right\rfloor, where XX is the total number of teams who submit the answer mn\frac{m}{n} (including your own team), and YY is the total number of teams who submit a valid answer. Otherwise, your score is 0 . (Your answer is not graded based on correctness, whether your fraction is in lowest terms, whether it is at most 1, etc.)

Answer, solutions and source