Found problems: 85335
Consider the maximum value of circular cone inscribed to a sphere, the ratio of it to the volume of the sphere is________.
Let $ p,q,n$ be three positive integers with $ p \plus{} q < n$. Let $ (x_{0},x_{1},\cdots ,x_{n})$ be an $ (n \plus{} 1)$-tuple of integers satisfying the following conditions :
(a) $ x_{0} \equal{} x_{n} \equal{} 0$, and
(b) For each $ i$ with $ 1\leq i\leq n$, either $ x_{i} \minus{} x_{i \minus{} 1} \equal{} p$ or $ x_{i} \minus{} x_{i \minus{} 1} \equal{} \minus{} q$.
Show that there exist indices $ i < j$ with $ (i,j)\neq (0,n)$, such that $ x_{i} \equal{} x_{j}$.
Let be given three quadratic polynomials:
$P_1(x) = x^2 + p_1x+q_1, P_2(x) = x^2+ p_2x+q_2, P_3(x) = x^2 + p_3x+q_3$.
Prove that the equation $|P_1(x)|+|P_2(x)| = |P_3(x)|$ has at most eight real roots.
Let $ P$ be a point inside a regular tetrahedron $ T$ of unit volume. The four planes passing through $ P$ and parallel to the faces of $ T$ partition $ T$ into 14 pieces. Let $ f(P)$ be the joint volume of those pieces that are neither a tetrahedron nor a parallelepiped (i.e., pieces adjacent to an edge but not to a vertex). Find the exact bounds for $ f(P)$ as $ P$ varies over $ T.$
In a deck of $n > 1$ cards, some digits from $1$ to$8$are written on each card.
A digit may occur more than once, but at most once on a certain card.
On each card at least one digit is written, and no two cards are denoted
by the same set of digits. Suppose that for every $k=1,2,\dots,7$ digits, the
number of cards that contain at least one of them is even. Find $n$.
Find the smallest volume bounded by the coordinate planes and by a tangent plane to the ellipsoid
$$\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}+\frac{z^{2}}{c^{2}}=1.$$
In a group of mathematicians everybody has at least one friend (friendship is a symmetric relation). Show that there is a mathematician all of whose friends have average number of friends not smaller than the average number of friends in the whole group.
The positive integer equal to the expression
\[ \sum_{i=0}^{9} \left(i+(-9)^i\right)8^{9-i} \binom{9}{i}\]
is divisible by exactly six distinct primes. Find the sum of these six distinct prime factors.
[i]Team #7[/i]
Let $ v$, $ w$, $ x$, $ y$, and $ z$ be the degree measures of the five angles of a pentagon. Suppose $ v < w < x < y < z$ and $ v$, $ w$, $ x$, $ y$, and $ z$ form an arithmetic sequence. Find the value of $ x$.
$ \textbf{(A)}\ 72 \qquad \textbf{(B)}\ 84 \qquad \textbf{(C)}\ 90 \qquad \textbf{(D)}\ 108 \qquad \textbf{(E)}\ 120$
A sequence of integers $a_1$, $a_2$, $a_3$, $\ldots$ is chosen so that $a_n = a_{n - 1} - a_{n - 2}$ for each $n \ge 3$. What is the sum of the first 2001 terms of this sequence if the sum of the first 1492 terms is 1985, and the sum of the first 1985 terms is 1492?
In the [i]magic square[/i] below, every integer from $1$ to $25$ can be filled in such that the sum in every row, column, and long diagonal is the same. Given that the number in the center square is $18$, what is the sum of the entries in the shaded squares?
[asy]
size(4cm);
for (int i = 0; i <= 5; ++i) {
draw((0,i)--(5,i));
}
for (int i = 0; i <= 5; ++i) {
draw((i,0)--(i,5));
}
for (int i = 0; i <= 4; ++i) {
for (int j = 0; j <= 4; ++j) {
if ((i+j)%6 == 1 || (i-j)%6 == 3) {
fill((i,j)--(i+1,j)--(i+1,j+1)--(i,j+1)--cycle, gray);
}
}
}
label("\Large{18}", (2.5,2.5));
[/asy]
[i]2017 CCA Math Bonanza Individual Round #5[/i]
The area of a convex polygon in the plane is equally shared by the four standard quadrants, and all non-zero lattice points lie outside the polygon. Show that the area of the polygon is less than $4$.
Consider the functions $f,g,h:\mathbb{R}_{\geqslant 0}\to\mathbb{R}_{\geqslant 0}$ and the binary operation $*:\mathbb{R}_{\geqslant 0}\times \mathbb{R}_{\geqslant 0}\to \mathbb{R}_{\geqslant 0}$ defined as \[x*y=f(x)+g(y)+h(x)\cdot|x-y|,\]for all $x,y\in\mathbb{R}_{\geqslant 0}$. Suppose that $(\mathbb{R}_{\geqslant 0},*)$ is a commutative monoid. Determine the functions $f,g,h$.
There are $n$ aborigines on an island. Any two of them are either friends or enemies. One day, the chieftain orders that all
citizens (including himself) make and wear a necklace with zero or more stones so that:
(i) given a pair of friends, there exists a color such that each has a stone of that color;
(ii) given a pair of enemies,there does not exist a color such that each a stone of that color.
(a) Prove that the aborigines can carry out the chieftain’s order.
(b) What is the minimum number of colors of stones required for the
aborigines to carry out the chieftain’s order?
In triangle $\triangle ABC$ points $M,N$ lie on $BC$ st : $\angle BAM= \angle MAN= \angle NAC$ . Points $P,Q$ are on the angle bisector of $BAC$, on the same side of $BC$ as A , st :
$$\frac{1}{3} \angle BAC = \frac{1}{2} \angle BPC = \angle BQC$$
Let $E = AM \cap CQ$ and $F = AN \cap BQ$ . Prove that the common tangents to $(EPF), (EQF)$ and the circumcircle of $\triangle ABC$ , are concurrent.
Given the polynomial $p(x) = x^2 + x - 70$, do there exist integers $0<m<n$, so that $p(m)$ is divisible by $n$ and $p(m+1)$ is divisible by $n+1$?
[i]Proposed by Nairy Sedrakyan[/i]
If $x<0$, then $\left|x-\sqrt{(x-1)^2}\right|$ equals
$\textbf{(A) }1\qquad\textbf{(B) }1-2x\qquad\textbf{(C) }-2x-1\qquad\textbf{(D) }1+2x\qquad \textbf{(E) }2x-1$
Find the number of permutations of the letters $AAAABBBCC$ where no letter is next to another letter of the same type. For example, count $ABCABCABA$ and $ABABCABCA$ but not $ABCCBABAA$.
There are villager $0$, villager $1$, . . . , villager $2015$ i.e. $2016$ people in the village. You are villager $0$. Each villager is either honest or liar. You don’t know each villager is honest or liar, but you know you are honest and the number of liar is equal or smaller than integer $T$.
The villagers became to write one letter without fail from one day. For integers $1 \le n \le 2015$, there are set integers $1 < k_n < 2015$. The letter villager $i$ wrote in day $n$ of the morning is delivered to villager $i + k_n$ if villager $i$ is honest, or villager $i - k_n$ if villager $i$ is liar in day $n$ of the evening. If $i - j$ is divisible by $2016$, villager $i$ and $j$ point same villager. Villagers don’t know $k_n$, but sender is told when letters are received. Villager can write anything on a letter, and each villager receives letters from any villagers a sufficient number of times after enough time. i.e. there are $n$ satisfying $k = k_n$ infinitely for each integer $1 \le k \le 2015$.
You want to know the honest persons of this village. You can gather all villagers just once and instruct in one day of noon. The honest person obeys your instruction but the liar person not always obeys and he or she writes on a letter anything possible.
One day from your instruction for a while, you could determine all honest persons of this village. Find the maximum value of $T$ such that it is possible to do this if you instruct appropriate regardless of the villagers who are honest or liar.
If five boys and three girls are randomly divided into two four-person teams, what is the probability that all three girls will end up on the same team?
$\text{(A) }\frac{1}{7}\qquad\text{(B) }\frac{2}{7}\qquad\text{(C) }\frac{1}{10}\qquad\text{(D) }\frac{1}{14}\qquad\text{(E) }\frac{1}{28}$
In a triangle $ABC,$ incircle touches the sides $BC, CA, AB$ at $D, E, F,$ respectively. A circle $\omega$ passing through $A$ and tangent to line $BC$ at $D$ intersects the line segments $BF$ and $CE$ at $K$ and $L,$ respectively. The line passing through $E$ and parallel to $DL$ intersects the line passing through $F$ and parallel to $DK$ at $P.$ If $R_1, R_2, R_3, R_4$ denotes the circumradius of the triangles $AFD, AED, FPD, EPD,$ respectively, prove that $R_1R_4=R_2R_3.$
Two grasshoppers sit at opposite ends of the interval $[0, 1]$. A finite number of points (greater than zero) in the interval are marked. A move is for a grasshopper to select a marked point and jump over it to the equidistant point the other side. This point must lie in the interval for the move to be allowed, but it does not have to be marked. What is the smallest $n$ such that if each grasshopper makes $n$ moves or less, then they end up with no marked points between them?
In square $ABCD$ with side length $2$, let $P$ and $Q$ both be on side $AB$ such that $AP=BQ=\frac{1}{2}$. Let $E$ be a point on the edge of the square that maximizes the angle $PEQ$. Find the area of triangle $PEQ$.
About pentagon $ABCDE$ is known that angle $A$ and angle $C$ are right and that the sides $| AB | = 4$, $| BC | = 5$, $| CD | = 10$, $| DE | = 6$. Furthermore, the point $C'$ that appears by mirroring $C$ in the line $BD$, lies on the line segment $AE$. Find angle $E$.
Let $P(x)=x^n +a_1x^{n-1}+a_2x^{n-2}+\ldots+a_{n-1}x+a_n$ be a polynomial of degree $n$ and $n$ real roots, all of them in the interval $(0,1)$. Prove that for all $k=\overline{1,n}$ the following inequality holds:
\[(-1)^k(a_k+a_{k+1}+\ldots+a_n)>0.\]
[i]Proposed by N. Safaei (Iran)[/i]