Found problems: 85335
Let $ABCD$ be a convex quadrilateral with $\angle B= \angle D$. Prove that the midpoint of $BD$ lies on the common internal tangent to the incircles of triangles $ABC$ and $ACD$.
As shown on the figure, square $PQRS$ is inscribed in right triangle $ABC$, whose right angle is at $C$, so that $S$ and $P$ are on sides $BC$ and $CA$, respectively, while $Q$ and $R$ are on side $AB$. Prove that $A B\geq3QR$ and determine when equality occurs.
[asy]
defaultpen(linewidth(0.7)+fontsize(10));
size(150);
real a=8, b=6;
real y=a/((a^2+b^2)/(a*b)+1), r=degrees((a,b))+180;
pair A=b*dir(-r)*dir(90), B=a*dir(180)*dir(-r), C=origin, S=y*dir(-r)*dir(180), P=(y*b/a)*dir(90-r), Q=foot(P, A, B), R=foot(S, A, B);
draw(A--B--C--cycle^^R--S--P--Q);
pair point=incenter(A,B,C);
label("$A$", A, dir(point--A));
label("$B$", B, dir(point--B));
label("$C$", C, dir(point--C));
label("$S$", S, dir(point--S));
label("$R$", R, dir(270));
label("$Q$", Q, dir(270));
label("$P$", P, dir(point--P));[/asy]
For what largest $n$ are there $n$ seven-digit numbers that are successive members of one geometric progression?
Find the sum of the elements of the set
$$M = \left\{ \frac{n}{2}+\frac{m}{5} \,\, | m, n = 0, 1, 2,..., 100\right\}$$
Does there exist a subset $M$ of positive integers such that for all positive rational numbers $r<1$ there exists exactly one finite subset of $M$ like $S$ such that sum of reciprocals of elements in $S$ equals $r$.
You are trying to maximize a function of the form $f(x, y, z) = ax + by + cz$, where $a$, $b$, and $c$ are constants. You know that $f(3, 1, 1) > f(2, 1, 1)$, $f(2, 2, 3) > f(2, 3, 4)$, and $f(3, 3, 4) > f(3, 3, 3)$. For $-5 \le x,y,z \le 5$, what value of $(x,y,z)$ maximizes the value of $f(x, y, z)$? Give your answer as an ordered triple.
Suppose $ABC$ is a triangle with circumcenter $O$. Point $D$ is the reflection of $A$ with respect to $BC$. Suppose $\ell$ is the line which is parallel to $BC$ and passes through $O$. The line through $B$ and parallel to $CD$ meets $\ell$ at $B_1$. Lines $CB_1$ and $BD$ intersect at point $B_2$. The line through $C$ parallel to $BD$ and $\ell$ meet at $C_1$. Finally, $BC_1$ and $CD$ intersects at point $C_2$. Prove that points $A, B_2, C_2, D$ lie on a circle.
Let $n >1$ be a natural number and $T_{n}(x)=x^n + a_{n-1}x^{n-1} + a_{n-2}x^{n-2} + ... + a_1 x^1 + a_0$.\\
Assume that for each nonzero real number $t $ we have $T_{n}(t+\frac {1}{t})=t^n+\frac {1}{t^n} $.\\
Prove that for each $0\le i \le n-1 $
$gcd (a_i,n) >1$.
[i]Proposed by Morteza Saghafian[/i]
Nine fair coins are flipped independently and placed in the cells of a $3$ by $3$ square grid. Let $p$ be the probability that no row has all its coins showing heads and no column has all its coins showing tails. If $p=\frac ab$ for relatively prime positive integers $a$ and $b$, compute $100a+b$.
Solve : $y(x+y)^2=9 $ ; $y(x^3-y^3)=7$
Let $ A $ be a ring that is not a division ring, and such that any non-unit of it is idempotent. Show that:
[b]a)[/b] $ \left( U(A) +A\setminus\left( U(A)\cup \{ 0\} \right) \right)\cap U(A) =\emptyset . $
[b]b)[/b] Every element of $ A $ is idempotent.
Let $n\ge 3$ be a fixed integer. There are $m\ge n+1$ beads on a circular necklace. You wish to paint the beads using $n$ colors, such that among any $n+1$ consecutive beads every color appears at least once. Find the largest value of $m$ for which this task is $\emph{not}$ possible.
[i]Carl Schildkraut, USA[/i]
Find all pairs of integers $(a,b)$ such that
$(b^2+7(a-b))^2=a^{3}b$.
$D_n$ is a set of domino pieces. For each pair of non-negative integers $(a, b)$ with $a \le b \le n$, there is one domino, denoted $[a, b]$ or $[b, a]$ in $D_n$. A [i]ring [/i] is a sequence of dominoes $[a_1, b_1], [a_2, b_2], ... , [a_k, b_k]$ such that $b_1 = a_2, b_2 = a_3, ... , b_{k-1} = a_k$ and $b_k = a_1$. Show that if $n$ is even there is a ring which uses all the pieces. Show that for n odd, at least $(n+1)/2$ pieces are not used in any ring. For $n$ odd, how many different sets of $(n+1)/2$ are there, such that the pieces not in the set can form a ring?
Find all surjective functions $ f: \mathbb{N} \to \mathbb{N}$ such that for every $ m,n \in \mathbb{N}$ and every prime $ p,$ the number $ f(m + n)$ is divisible by $ p$ if and only if $ f(m) + f(n)$ is divisible by $ p$.
[i]Author: Mohsen Jamaali and Nima Ahmadi Pour Anari, Iran[/i]
Kevin writes a nonempty subset of $S = \{ 1, 2, \dots 41 \}$ on a board. Each day, Evan takes the set last written on the board and decreases each integer in it by $1.$ He calls the result $R.$ If $R$ does not contain $0$ he writes $R$ on the board. If $R$ contains $0$ he writes the set containing all elements of $S$ not in $R$. On Evan's $n$th day, he sees that he has written Kevin's original subset for the $1$st time. Find the sum of all possible $n.$
Given a prime $p$, show that \[\left(1+p\sum_{k=1}^{p-1}k^{-1}\right)^2 \equiv 1-p^2\sum_{k=1}^{p-1}k^{-2} \pmod{p^4}.\]
[i]Timothy Chu.[/i]
Here is a problem we (me and my colleagues) suggested and was given at the competition this year. The problem statement is very natural and short. However, we have not seen such a problem before.
A real $2024 \times 2024$ matrix $A$ is called nice if $(Av, v) = 1$ for every vector $v\in \mathbb{R}^{2024}$ with unit norm.
a) Prove that the only nice matrix such that all of its eigenvalues are real is the identity matrix.
b) Find an example of a nice non-identity matrix
Positive integers $k, m, n$ satisfy $mn=k^2+k+3$, prove that at least one of the equations $x^2+11y^2=4m$ and $x^2+11y^2=4n$ has an odd solution.
Let $m, n, k > 1$ be positive integers. For a set $S$ of positive integers, define $S(i,j)$ for $i<j$ to be the number of elements in $S$ strictly between $i$ and $j$. We say two sets $(X,Y)$ are a [i]fat[/i] pair if \[ X(i,j)\equiv Y(i,j) \pmod{n} \] for every $i,j \in X \cap Y$. (In particular, if $\left\lvert X \cap Y \right\rvert < 2$ then $(X,Y)$ is fat.)
If there are $m$ distinct sets of $k$ positive integers such that no two form a fat pair, show that $m<n^{k-1}$.
[i]Proposed by Allen Liu[/i]
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:
$\bullet$ Lucky looks at the coin (or lack thereof) underneath him.
$\bullet \, - \, $ If the coin is heads, Lucky flips it to tails up, turns around, and steps forward a distance of one unit.
$ \qquad a -$ If the coin is tails, Lucky picks up the coin and steps forward a distance of one unit facing the same direction.
$ \qquad a -$ 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?
In a convex quadrilateral $ ABCD$ the points $ P$ and $ Q$ are chosen on the sides $ BC$ and $ CD$ respectively so that $ \angle{BAP}\equal{}\angle{DAQ}$. Prove that the line, passing through the orthocenters of triangles $ ABP$ and $ ADQ$, is perpendicular to $ AC$ if and only if the triangles $ ABP$ and $ ADQ$ have the same areas.
Tweaking a convex $n$-gon means the following: choose two adjacent sides $AB$ and $BC$ and replaces them with the line segment $AM$, $MN$, $NC$, where $M \in AB$ and $N \in BC$ are arbitrary points inside these segments. In other words, you cut off a corner and get an $(n+1)$-corner.
Starting from a regular hexagon $P_6$ with area $1$, by continuous Tweaks a sequence $P_6,P_7,P_8, ...$ convex polygons. Show that Area of $P_n$ for all $n\ge 6$ greater than $\frac1 2$ is, regardless of how tweaks takes place.
In acute triangle $ABC (AB>AC)$, $M$ is the midpoint of minor arc $BC$, $O$ is the circumcenter of $(ABC)$ and $AK$ is its diameter. The line parallel to $AM$ through $O$ meets segment $AB$ at $D$, and $CA$ extended at $E$. Lines $BM$ and $CK$ meet at $P$, lines $BK$ and $CM$ meet at $Q$. Prove that $\angle OPB+\angle OEB =\angle OQC+\angle ODC$.
Let $ABC$ be a triangle with $AB = 13$, $BC = 14$, $CA = 15$. Let $H$ be the orthocenter of $ABC$. Find the radius of the circle with nonzero radius tangent to the circumcircles of $AHB$, $BHC$, $CHA$.