Navvye/Olympiad-Problems-CSV huggingface dataset:
2021 China Second Round
Let $k\ge 2$ be an integer and $a_1,a_2,\cdots,a_k$ be $k$ non-zero reals. Prove that there are finitely many pairs of pairwise distinct positive integers $(n_1,n_2,\cdots,n_k)$ such that

$$a_1\cdot n_1!+a_2\cdot n_2!+\cdots+a_k\cdot n_k!=0.$$
2021 China Second Round
In $\triangle ABC$, point $M$ is the middle point of $AC$. $MD//AB$ and meet the tangent of $A$ to $\odot(ABC)$ at point $D$. Point $E$ is in $AD$ and point $A$ is the middle point of $DE$. $\{P\}=\odot(ABE)\cap AC,\{Q\}=\odot(ADP)\cap DM$. Prove that $\angle QCB=\angle BAC$.
2021 China Second Round
If $n\ge 4,\ n\in\mathbb{N^*},\ n\mid (2^n-2)$. Prove that $\frac{2^n-2}{n}$ is not a prime number.
2021 China Second Round
Find the minimum value of $c$ such that for any positive integer $n\ge 4$ and any set $A\subseteq \{1,2,\cdots,n\}$, if $|A| >cn$, there exists a function $f:A\to\{1,-1\}$ satisfying

$$\left| \sum_{a\in A}a\cdot f(a)\right| \le 1.$$
2020 China Second Round Olympiad
In triangle $ABC,$ $AB=BC,$ and let $I$ be the incentre of $\triangle ABC.$ $M$ is the midpoint of segment $BI.$ $P$ lies on segment $AC,$ such that $AP=3PC.$ $H$ lies on line $PI,$ such that $MH\perp PH.$ $Q$ is the midpoint of the arc $AB$ of the circumcircle of $\triangle ABC$. Prove that $BH\perp QH.$
2020 China Second Round Olympiad
Let $n\geq3$ be a given integer, and let $a_1,a_2,\cdots,a_{2n},b_1,b_2,\cdots,b_{2n}$ be $4n$ nonnegative reals, such that $$a_1+a_2+\cdots+a_{2n}=b_1+b_2+\cdots+b_{2n}>0,$$and for any $i=1,2,\cdots,2n,$ $a_ia_{i+2}\geq b_i+b_{i+1},$ where $a_{2n+1}=a_1,$ $a_{2n+2}=a_2,$ $b_{2n+1}=b_1.$ Detemine the minimum of $a_1+a_2+\cdots+a_{2n}.$
2020 China Second Round Olympiad
Let $a_1=1,$ $a_2=2,$ $a_n=2a_{n-1}+a_{n-2},$ $n=3,4,\cdots.$ Prove that for any integer $n\geq5,$ $a_n$ has at least one prime factor $p,$ such that $p\equiv 1\pmod{4}.$
2020 China Second Round Olympiad
Given a convex polygon with 20 vertexes, there are many ways of traingulation it (as 18 triangles). We call the diagram of triangulation, meaning the 20 vertexes, with 37 edges(17 triangluation edges and the original 20 edges), a T-diagram. And the subset of this T-diagram with 10 edges which covers all 20 vertexes(meaning any two edges in the subset doesn't cover the same vertex) calls a "perfect matching" of this T-diagram. Among all the T-diagrams, find the maximum number of "perfect matching" of a T-diagram.
2019 China Second Round Olympiad
In acute triangle $\triangle ABC$, $M$ is the midpoint of segment $BC$. Point $P$ lies in the interior of $\triangle ABC$ such that $AP$ bisects $\angle BAC$. Line $MP$ intersects the circumcircles of $\triangle ABP,\triangle ACP$ at $D,E$ respectively. Prove that if $DE=MP$, then $BC=2BP$.
2019 China Second Round Olympiad
Let $a_1,a_2,\cdots,a_n$ be integers such that $1=a_1\le a_2\le \cdots\le a_{2019}=99$. Find the minimum $f_0$ of the expression $$f=(a_1^2+a_2^2+\cdots+a_{2019}^2)-(a_1a_3+a_2a_4+\cdots+a_{2017}a_{2019}),$$and determine the number of sequences $(a_1,a_2,\cdots,a_n)$ such that $f=f_0$.
2019 China Second Round Olympiad
Let $m$ be an integer where $|m|\ge 2$. Let $a_1,a_2,\cdots$ be a sequence of integers such that $a_1,a_2$ are not both zero, and for any positive integer $n$, $a_{n+2}=a_{n+1}-ma_n$.



Prove that if positive integers $r>s\ge 2$ satisfy $a_r=a_s=a_1$, then $r-s\ge |m|$.
2019 China Second Round Olympiad
Let $V$ be a set of $2019$ points in space where any of the four points are not on the same plane, and $E$ be the set of edges connected between them. Find the smallest positive integer $n$ satisfying the following condition: if $E$ has at least $n$ elements, then there exists $908$ two-element subsets of $E$ such that



The two edges in each subset share a common vertice,

Any of the two subsets do not intersect.
2019 China Second Round Olympiad
Suppose that $a_1,a_2,\cdots,a_{100}\in\mathbb{R}^+$ such that $a_i\geq a_{101-i}\,(i=1,2,\cdots,50).$

Let $x_k=\frac{ka_{k+1}}{a_1+a_2+\cdots+a_k}\,(k=1,2,\cdots,99).$ Prove that

$$x_1x_2^2\cdots x_{99}^{99}\leq 1.$$
2019 China Second Round Olympiad
Find all the positive integers $n$ such that:

$(1)$ $n$ has at least $4$ positive divisors.

$(2)$ if all positive divisors of $n$ are $d_1,d_2,\cdots ,d_k,$ then $d_2-d_1,d_3-d_2,\cdots ,d_k-d_{k-1}$ form a geometric sequence.
2019 China Second Round Olympiad
Point $A,B,C,D,E$ lie on a line in this order, such that $BC=CD=\sqrt{AB\cdot DE},$ $P$ doesn't lie on the line, and satisfys that $PB=PD.$ Point $K,L$ lie on the segment $PB,PD,$ respectively, such that $KC$ bisects $\angle BKE,$ and $LC$ bisects $\angle ALD.$

Prove that $A,K,L,E$ are concyclic.
2019 China Second Round Olympiad
Each side of a convex $2019$-gon polygon is dyed with red, yellow and blue, and there are exactly $673$ sides of each kind of color. Prove that there exists at least one way to draw $2016$ diagonals to divide the convex $2019$-gon polygon into $2017$ triangles, such that any two of the $2016$ diagonals don't have intersection inside the $2019$-gon polygon,and for any triangle in all the $2017$ triangles, the colors of the three sides of the triangle are all the same, either totally different.
2018 China Second Round Olympiad
Let $ a_1,a_2,\cdots,a_n,b_1,b_2,\cdots,b_n,A,B$  are positive reals such that $ a_i\leq b_i,a_i\leq A$ $(i=1,2,\cdots,n)$  and $\frac{b_1 b_2 \cdots b_n}{a_1 a_2 \cdots a_n}\leq \frac{B}{A}.$ Prove that$$\frac{(b_1+1) (b_2+1) \cdots (b_n+1)}{(a_1+1) (a_2+1) \cdots (a_n+1)}\leq \frac{B+1}{A+1}.$$
2018 China Second Round Olympiad
In triangle $\triangle ABC$, $AB<AC$, $M,D,E$ are the midpoints of $BC$, the arcs $BAC$ and $BC$ of the circumcircle of $\triangle ABC$ respectively. The incircle of $\triangle ABC$ touches $AB$ at $F$, $AE$ meets $BC$ at $G$, and the perpendicular to $AB$ at $B$ meets segment $EF$ at $N$. If $BN=EM$, prove that $DF$ is perpendicular to $FG$.
2018 China Second Round Olympiad
Let $n,k,m$ be positive integers, where $k\ge 2$ and $n\le m < \frac{2k-1}{k}n$. Let $A$ be a subset of $\{1,2,\ldots ,m\}$ with $n$ elements. Prove that every integer in the range $\left(0,\frac{n}{k-1}\right)$ can be expressed as $a-b$, where $a,b\in A$.
2018 China Second Round Olympiad
Define sequence $\{a_n\}$: $a_1$ is any positive integer, and for any positive integer $n\ge 1$, $a_{n+1}$ is the smallest positive integer coprime to $\sum_{i=1}^{n} a_i$ and not equal to $a_1,\ldots, a_n$. Prove that every positive integer appears in the sequence $\{a_n\}$.
2018 China Second Round Olympiad
Let $a,b \in \mathbb R,f(x)=ax+b+\frac{9}{x}.$ Prove that there exists $x_0 \in \left[1,9 \right],$ such that $|f(x_0)| \ge 2.$
2018 China Second Round Olympiad
In triangle $\triangle ABC, AB=AC.$ Let $D$ be on segment $AC$ and $E$ be a point on the extended line $BC$ such that $C$ is located between $B$ and $E$ and $\frac{AD}{DC}=\frac{BC}{2CE}$. Let $\omega$ be the circle with diameter $AB,$ and $\omega$ intersects segment $DE$ at $F.$ Prove that $B,C,F,D$ are concyclic.
2018 China Second Round Olympiad
Let set $A=\{1,2,\ldots,n\} ,$ and $X,Y$ be two subsets (not necessarily distinct) of $A.$ Define that $\textup{max} X$ and $\textup{min} Y$ represent the greatest element of $X$ and the least element of $Y,$ respectively. Determine the number of two-tuples $(X,Y)$ which satisfies $\textup{max} X>\textup{min} Y.$
2018 China Second Round Olympiad
Prove that for any integer $a \ge 2$ and positive integer $n,$ there exist positive integer $k$ such that $a^k+1,a^k+2,\ldots,a^k+n$ are all composite numbers.
2017 China Second Round Olympiad
Let $ x,y$ are  real numbers such that $x^2+2cosy=1$. Find the ranges of $x-cosy$.
2017 China Second Round Olympiad
Let $x_1,x_2,x_3\geq 0$ and $x_1+x_2+x_3=1$. Find the minimum value and the maximum value of $(x_1+3x_2+5x_3)\left(x_1+\frac{x_2}{3}+\frac{x_3}{5}\right).$
2017 China Second Round Olympiad
Given an isocleos triangle $ABC$ with equal sides $AB=AC$ and incenter $I$.Let $\Gamma_1$be the circle centered at $A$ with radius $AB$,$\Gamma_2$ be the circle centered at $I$ with radius $BI$.A circle $\Gamma_3$ passing through $B,I$ intersects $\Gamma_1$,$\Gamma_2$ again at $P,Q$ (different from $B$) respectively.Let $R$ be the intersection of $PI$ and $BQ$.Show that $BR \perp CR$.
2017 China Second Round Olympiad
Given a sequence $\{a_n\}$: $a_1=1, a_{n+1}=\left\{

    \begin{array}{lcr}

      a_n+n,\quad a_n\le n, \\
      a_n-n,\quad a_n>n,

    \end{array}

    \right.

\quad n=1,2,\cdots.$

Find the number of positive integers $r$ satisfying $a_r<r\le 3^{2017}$.
2017 China Second Round Olympiad
Each square of a $33\times 33$ square grid is colored in one of the three colors: red, yellow or blue, such that the numbers of squares in each color are the same. If two squares sharing a common edge are in different colors, call that common edge a separating edge. Find the minimal number of separating edges in the grid.
2017 China Second Round Olympiad
Let $m,n$ be integers greater than 1,$m \geq n$,$a_1,a_2,\dots,a_n$ are $n$ distinct numbers not exceed $m$,which are relatively primitive.Show that for any real $x$,there exists $i$ for which $||a_ix|| \geq \frac{2}{m(m+1)} ||x||$,where $||x||$ denotes the distance between $x$ and the nearest integer to $x$ .
2016 China Second Round Olympiad
Let $f(x)$ is an odd function on  $R$ , $f(1)=1$ and $f(\frac{x}{x-1})=xf(x)$ $(\forall x<0)$.

Find the value of  $f(1)f(\frac{1}{100})+f(\frac{1}{2})f(\frac{1}{99})+f(\frac{1}{3})f(\frac{1}{98})+\cdots +f(\frac{1}{50})f(\frac{1}{51}).$
2016 China Second Round Olympiad
Let $a_1, a_2, \ldots, a_{2016}$ be real numbers such that $9a_i\ge 11a^2_{i+1}$ $(i=,2,\cdots,2015)$.

Find the  maximum value of $(a_1-a^2_2)(a_2-a^2_3)\cdots (a_{2015}-a^2_{2016})(a_{2016}-a^2_{1}).$
2016 China Second Round Olympiad
Let $X,Y$ be two points which lies on the line $BC$ of $\triangle ABC(X,B,C,Y\text{lies in sequence})$ such that $BX\cdot AC=CY\cdot AB$, $O_1,O_2$ are the circumcenters of $\triangle ACX,\triangle ABY$, $O_1O_2\cap AB=U,O_1O_2\cap AC=V$. Prove that $\triangle AUV$ is a isosceles triangle.
2016 China Second Round Olympiad
Given $10$ points in the space such that each $4$ points are not lie on a plane. Connect some points with some segments such that there are no triangles or quadrangles. Find the maximum number of the segments.
2016 China Second Round Olympiad
Let $p>3$ and $p+2$ are  prime numbers,and define sequence

$$a_{1}=2,a_{n}=a_{n-1}+\lfloor \dfrac{pa_{n-1}}{n}\rfloor$$

show that:for any $n=3,4,\cdots,p-1$ have $$n|pa_{n-1}+1$$
2015 China Second Round Olympiad
Let $a_1, a_2, \ldots, a_n$ be real numbers.Prove that you can select $\varepsilon _1, \varepsilon _2, \ldots, \varepsilon _n\in\{-1,1\}$ such that$$\left( \sum_{i=1}^{n}a_{i}\right)^2 +\left( \sum_{i=1}^{n}\varepsilon _ia_{i}\right)^2 \leq(n+1)\left( \sum_{i=1}^{n}a^2_{i}\right).$$
2015 China Second Round Olympiad
Let $S=\{A_1,A_2,\ldots ,A_n\}$, where $A_1,A_2,\ldots ,A_n$ are $n$ pairwise distinct finite sets $(n\ge 2)$, such that for any $A_i,A_j\in S$, $A_i\cup A_j\in S$. If $k= \min_{1\le i\le n}|A_i|\ge 2$, prove that there exist $x\in \bigcup_{i=1}^n A_i$, such that $x$ is in at least $\frac{n}{k}$ of the sets $A_1,A_2,\ldots ,A_n$ (Here $|X|$ denotes the number of elements in finite set $X$).
2015 China Second Round Olympiad
$P$ is a point on arc $\overarc{BC}$ of the circumcircle of $\triangle ABC$ not containing $A$, $K$ lies on segment $AP$ such that $BK$ bisects $\angle ABC$. The circumcircle of $\triangle KPC$ meets $AC,BD$ at $D,E$ respectively. $PE$ meets $AB$ at $F$. Prove that $\angle ABC=2\angle FCB$.
2015 China Second Round Olympiad
Find all positive integers $k$ such that for any positive integer $n$, $2^{(k-1)n+1}$ does not divide $\frac{(kn)!}{n!}$.
2015 China Second Round Olympiad
Let $a,b,c$ be  nonnegative real numbers.Prove that$$\frac{(a-bc)^2+(b-ca)^2+(c-ab)^2}{(a-b)^2+(b-c)^2+(c-a)^2}\geq\frac{1}{2}.$$
2015 China Second Round Olympiad
In isoceles $\triangle ABC$, $AB=AC$, $I$ is its incenter, $D$ is a point inside $\triangle ABC$ such that $I,B,C,D$ are concyclic. The line through $C$ parallel to $BD$ meets $AD$ at $E$. Prove that $CD^2=BD\cdot CE$.
2015 China Second Round Olympiad
Prove that there exist infinitely many positive integer triples $(a,b,c)(a,b,c>2015)$ such that

$$ a|bc-1, b|ac+1, c|ab+1.$$
2015 China Second Round Olympiad
Given positive integers $m,n(2\le m\le n)$, let $a_1,a_2,\ldots ,a_m$ be a permutation of any $m$ pairwise distinct numbers taken from $1,2,\ldots ,n$. If there exist $k\in\{1,2,\ldots ,m\}$ such that $a_k+k$ is odd, or there exist positive integers $k,l(1\le k<l\le m)$ such that $a_k>a_l$, then call $a_1,a_2,\ldots ,a_m$ a good sequence. Find the number of good sequences.
2014 China Second Round Olympiad
Let $a,b,c$ be real numbers such that $a+b+c=1$ and $abc>0$ . Prove that$$bc+ca+ab<\frac{\sqrt{abc}}{2}+\frac{1}{4}.$$
2014 China Second Round Olympiad
Let $ABC$ be an acute triangle such that $\angle BAC \neq 60^\circ$. Let $D,E$ be points such that $BD,CE$ are tangent to the circumcircle of $ABC$ and $BD=CE=BC$ ($A$ is on one side of line $BC$ and $D,E$ are on the other side). Let $F,G$ be intersections of line $DE$ and lines $AB,AC$. Let $M$ be intersection of $CF$ and $BD$, and $N$ be intersection of $CE$ and $BG$. Prove that $AM=AN$.
2014 China Second Round Olympiad
Let $S=\{1,2,3,\cdots,100\}$. Find the maximum value of integer $k$, such that there exist $k$ different nonempty subsets of $S$ satisfying the condition: for any two of the $k$ subsets, if their intersection is nonemply, then the minimal element of their intersection is not equal to the maximal element of either of the two subsets.
2014 China Second Round Olympiad
Let $x_1,x_2,\dots,x_{2014}$ be integers among which no two are congurent modulo $2014$. Let $y_1,y_2,\dots,y_{2014}$ be integers among which no two are congurent modulo $2014$. Prove that one can rearrange $y_1,y_2,\dots,y_{2014}$ to $z_1,z_2,\dots,z_{2014}$, so that among $$x_1+z_1,x_2+z_2,\dots,x_{2014}+z_{2014}$$ no two are congurent modulo $4028$.
2013 China Second Round Olympiad
$AB$ is a chord of circle $\omega$, $P$ is a point on minor arc $AB$, $E,F$ are on segment $AB$ such that $AE=EF=FB$. $PE,PF$ meets $\omega$ at $C,D$ respectively. Prove that $EF\cdot CD=AC\cdot BD$.
2013 China Second Round Olympiad
Let $u,v$ be positive integers. Define sequence $\{a_n\}$ as follows: $a_1=u+v$, and for integers $m\ge 1$,



$$\begin{array}{lll}

\begin{cases}

a_{2m}=a_m+u, \\
a_{2m+1}=a_m+v, 

\end{cases}

\end{array}$$

Let $S_m=a_1+a_2+\ldots +a_m(m=1,2,\ldots )$. Prove that there are infinitely many perfect squares in the sequence $\{S_n\}$.
2013 China Second Round Olympiad
$n$ students take a test with $m$ questions, where $m,n\ge 2$ are integers. The score given to every question is as such: for a certain question, if $x$ students fails to answer it correctly, then those who answer it correctly scores $x$ points, while those who answer it wrongly scores $0$. The score of a student is the sum of his scores for the $m$ questions. Arrange the scores in descending order $p_1\ge p_2\ge \ldots \ge p_n$. Find the maximum value of $p_1+p_n$.
2013 China Second Round Olympiad
Let $n,k$ be integers greater than $1$, $n<2^k$. Prove that there exist $2k$ integers none of which are divisible by $n$, such that no matter how they are separated into two groups there exist some numbers all from the same group whose sum is divisible by $n$.
2013 China Second Round Olympiad
For any positive integer $n$ , Prove that  there is not exist  three odd integer $x,y,z$ satisfing the equation  $(x+y)^n+(y+z)^n=(x+z)^n$.
2013 China Second Round Olympiad
Let $n$ be a positive  odd integer , $a_1,a_2,\cdots,a_n$ be any permutation of the positive integers $1,2,\cdots,n$ . Prove that :$(a_1-1)(a^2_2-2)(a^3_3-3)\cdots (a^n_n-n)$ is an even number.
2013 China Second Round Olympiad
The integers $n>1$ is given . The positive integer $a_1,a_2,\cdots,a_n$ satisfing condition :

(1) $a_1<a_2<\cdots<a_n$;

(2) $\frac{a^2_1+a^2_2}{2},\frac{a^2_2+a^2_3}{2},\cdots,\frac{a^2_{n-1}+a^2_n}{2}$  are all perfect squares .

Prove that :$a_n\ge 2n^2-1.$
2012 China Second Round Olympiad
Let $P$ be a point on the graph of the function $y=x+\frac{2}{x}(x>0)$. $PA,PB$ are perpendicular to line $y=x$ and $x=0$, respectively, the feet of perpendicular being $A$ and $B$. Find the value of $\overrightarrow{PA}\cdot \overrightarrow{PB}$.
2012 China Second Round Olympiad
In $\triangle ABC$, the corresponding sides of angle $A,B,C$ are $a,b,c$ respectively. If $a\cos B-b\cos A=\frac{3}{5}c$, find the value of $\frac{\tan A}{\tan B}$.
2012 China Second Round Olympiad
Suppose that $x,y,z\in [0,1]$. Find the maximal value of the expression

$$\sqrt{|x-y|}+\sqrt{|y-z|}+\sqrt{|z-x|}.$$
2012 China Second Round Olympiad
Let $F$ be the focus of parabola $y^2=2px(p>0)$, with directrix $l$ and two points $A,B$ on it. Knowing that $\angle AFB=\frac{\pi}{3}$, find the maximal value of $\frac{|MN|}{|AB|}$, where $M$ is the midpoint of $AB$ and $N$ is the projection of $M$ to $l$.
2012 China Second Round Olympiad
Suppose two regular pyramids with the same base $ABC$: $P-ABC$ and $Q-ABC$ are circumscribed by the same sphere. If the angle formed by one of the lateral face and the base of pyramid $P-ABC$ is $\frac{\pi}{4}$, find the tangent value of the angle formed by one of the lateral face and the base of the pyramid $Q-ABC$.
2012 China Second Round Olympiad
Let $f(x)$ be an odd function on $\mathbb{R}$, such that $f(x)=x^2$ when $x\ge 0$. Knowing that for all $x\in [a,a+2]$, the inequality $f(x+a)\ge 2f(x)$ holds, find the range of real number $a$.
2012 China Second Round Olympiad
Find the sum of all integers $n$ satisfying the following inequality:

$$\frac{1}{4}<\sin\frac{\pi}{n}<\frac{1}{3}.$$
2012 China Second Round Olympiad
There are $4$ distinct codes used in an intelligence station, one of them applied in each week. No two codes used in two adjacent weeks are the same code. Knowing that code $A$ is used in the first week, find the probability that code $A$ is used in the seventh week.
2012 China Second Round Olympiad
Given a function $f(x)=a\sin x-\frac{1}{2}\cos 2x+a-\frac{3}{a}+\frac{1}{2}$, where $a\in\mathbb{R}, a\ne 0$.

(1) If for any $x\in\mathbb{R}$, inequality $f(x)\le 0$ holds, find all possible value of $a$.

(2) If $a\ge 2$, and there exists $x\in\mathbb{R}$, such that $f(x)\le 0$. Find all possible value of $a$.
2012 China Second Round Olympiad
Given a sequence $\{a_n\}$ whose terms are non-zero real numbers. For any positive integer $n$, the equality

$$(\sum_{i=1}^{n}a_i)^2=\sum_{i=1}^{n}a_i^3$$

holds.

(1) If $n=3$, find all possible sequence $a_1,a_2,a_3$;

(2) Does there exist such a sequence $\{a_n\}$ such that $a_{2011}=-2012$?
2012 China Second Round Olympiad
In the Cartesian plane $XOY$, there is a rhombus $ABCD$ whose side lengths are all $4$ and $|OB|=|OD|=6$, where $O$ is the origin.

(1) Prove that $|OA|\cdot |OB|$ is a constant.

(2) Find the locus of $C$ if $A$ is a point on the semicircle

$$(x-2)^2+y^2=4 \quad (2\le x\le 4).$$
2012 China Second Round Olympiad
In an acute-angled triangle $ABC$, $AB>AC$. $M,N$ are distinct points on side $BC$ such that $\angle BAM=\angle CAN$. Let $O_1,O_2$ be the circumcentres of $\triangle ABC, \triangle AMN$, respectively. Prove that $O_1,O_2,A$ are collinear.
2012 China Second Round Olympiad
Prove that the set $\{2,2^2,\ldots,2^n,\ldots\}$ satisfies the following properties:

(1) For every $a\in A, b\in\mathbb{N}$, if $b<2a-1$, then $b(b+1)$ isn't a multiple of $2a$;

(2) For every positive integer $a\notin A,a\ne 1$, there exists a positive integer $b$, such that $b<2a-1$ and $b(b+1)$ is a multiple of $2a$.
2012 China Second Round Olympiad
Let $P_0 ,P_1 ,P_2 , ... ,P_n$ be $n+1$ points in the plane. Let $d$($d>0$) denote the minimal value of all the distances between any two points. Prove that

$$|P_0P_1|\cdot |P_0P_2|\cdot ... \cdot |P_0P_n|>(\frac{d}{3})^n\sqrt{(n+1)!}.$$
2012 China Second Round Olympiad
Let $S_n=1+\frac{1}{2}+\frac{1}{3}+\ldots+\frac{1}{n}$, where $n$ is a positive integer. Prove that for any real numbers $a,b,0\le a\le b\le 1$, there exist infinite many $n\in\mathbb{N}$ such that

$$a<S_n-[S_n]<b$$

where $[x]$ represents the largest integer not exceeding $x$.
2011 China Second Round Olympiad
Let $P,Q$ be the midpoints of diagonals $AC,BD$ in cyclic quadrilateral $ABCD$. If $\angle BPA=\angle DPA$, prove that $\angle AQB=\angle CQB$.
2011 China Second Round Olympiad
For any integer $n\ge 4$, prove that there exists a $n$-degree polynomial $f(x)=x^n+a_{n-1}x^{n-1}+\cdots+a_0$

satisfying the two following properties:



(1) $a_i$ is a positive integer for any $i=0,1,\ldots,n-1$, and



(2) For any two positive integers $m$ and $k$ ($k\ge 2$) there exist distinct positive integers $r_1,r_2,...,r_k$, such that $f(m)\ne\prod_{i=1}^{k}f(r_i)$.
2011 China Second Round Olympiad
Given $n\ge 4$ real numbers $a_{n}>...>a_{1} > 0$. For $r > 0$, let $f_{n}(r)$ be the number of triples $(i,j,k)$ with $1\leq i<j<k\leq n$ such that $\frac{a_{j}-a_{i}}{a_{k}-a_{j}}=r$. Prove that ${f_{n}(r)}<\frac{n^{2}}{4}$.
2011 China Second Round Olympiad
Let $A$ be a $3 \times 9$ matrix. All elements of $A$ are positive integers. We call an $m\times n$ submatrix of $A$ "ox" if the sum of its elements is divisible by $10$, and we call an element of $A$ "carboxylic" if it is not an element of any "ox" submatrix. Find the largest possible number of "carboxylic" elements in $A$.
2011 China Second Round Olympiad
Let the set $A=(a_{1},a_{2},a_{3},a_{4})$ . If the sum of elements in every 3-element subset of $A$ makes up the set $B=(-1,5,3,8)$ , then find the set $A$.
2011 China Second Round Olympiad
Find the range of the function $f(x)=\frac{\sqrt{x^2+1}}{x-1}$.
2011 China Second Round Olympiad
Let $a,b$ be positive reals such that $\frac{1}{a}+\frac{1}{b}\leq2\sqrt2$ and $(a-b)^2=4(ab)^3$. Find $\log_a b$.
2011 China Second Round Olympiad
If ${\cos^5 x}-{\sin^5 x}<7({\sin^3 x}-{\cos ^3 x}) $ (for $x\in [  0,2\pi) $), then find the range of $x$.
2011 China Second Round Olympiad
We want to arrange $7$ students to attend $5$ sports events, but students $A$ and $B$ can't take part in the same event, every event has its own participants, and every student can only attend one event. How many arrangements are there?
2011 China Second Round Olympiad
In a tetrahedral $ABCD$, given that $\angle ADB=\angle BDC =\angle CDA=\frac{\pi}{3}$, $AD=BD=3$, and $CD=2$. Find the radius of the circumsphere of $ABCD$.
2011 China Second Round Olympiad
The line $x-2y-1=0$ insects the parabola $y^2=4x$ at two different points $A, B$. Let $C$ be a point on the parabola such that $\angle ACB=\frac{\pi}{2}$. Find the coordinate of point $C$.
2011 China Second Round Olympiad
Given that $a_{n}= \binom{200}{n} \cdot 6^{\frac{200-n}{3}} \cdot (\dfrac{1}{\sqrt{2}})^n$ ($ 1 \leq n \leq 95$). How many integers are there in the sequence $\{a_n\}$?
2011 China Second Round Olympiad
Let $f(x)=|\log(x+1)|$ and let $a,b$ be two real numbers ($a<b$) satisfying the equations $f(a)=f\left(-\frac{b+1}{a+1}\right)$ and $f\left(10a+6b+21\right)=4\log 2$. Find $a,b$.
2011 China Second Round Olympiad
A sequence $a_n$ satisfies $a_1 =2t-3$ ($t \ne 1,-1$), and $a_{n+1}=\dfrac{(2t^{n+1}-3)a_n+2(t-1)t^n-1}{a_n+2t^n-1}$.





i) Find $a_n$,



ii) If $t>0$, compare $a_{n+1}$ with $a_n$.
2011 China Second Round Olympiad
A line $\ell$ with slope of $\frac{1}{3}$ insects the ellipse $C:\frac{x^2}{36}+\frac{y^2}{4}=1$ at points $A,B$ and the point $P\left(  3\sqrt{2} , \sqrt{2}\right)$ is above the line $\ell$.





(1) Prove that the locus of the incenter of triangle $PAB$ is a segment,



(2) If $\angle APB=\frac{\pi}{3}$, then find the area of triangle $PAB$.
2010 China Second Round Olympiad
Given an acute triangle whose circumcenter is $O$.let $K$ be a point on $BC$,different from its midpoint.$D$ is on the extension of segment $AK,BD$ and $AC$,$CD$and$AB$intersect at $N,M$ respectively.prove that $A,B,D,C$ are concyclic.
2010 China Second Round Olympiad
Given a fixed integer $k>0,r=k+0.5$,define

$f^1(r)=f(r)=r[r],f^l(r)=f(f^{l-1}(r))(l>1)$

where $[x]$ denotes the smallest integer not less than $x$.

prove that there exists integer $m$ such that $f^m(r)$ is an integer.