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Home » Olympiad Resources » Iran » National Math Olympiad (3rd Round) » 2005

Iran National Math Olympiad (3rd Round) 2005

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Day 1

1	Suppose $a,b,c\in \mathbb R^+$ . Prove that : $(\frac ab+\frac bc+\frac ca)^2\geq (a+b+c)(\frac1a+\frac1b+\frac1c)$	S
2	Suppose $\{x_n\}$ is a decreasing sequence that $\displaystyle\lim_{n \rightarrow\infty}x_n=0$ . Prove that $\sum(-1)^nx_n$ is convergent	S
3	Find all $\alpha>0$ and $\beta>0$ that for each $(x_1,\dots,x_n)$ and $(y_1,\dots,y_n)\in\mathbb {R^+}^n$ that: $(\sum x_i^\alpha)(\sum y_i^\beta)\geq\sum x_iy_i$	S
4	Suppose $P,Q\in \mathbb R[x]$ that $deg\ P=deg\ Q$ and $PQ'-QP'$ has no real root. Prove that for each $\lambda \in \mathbb R$ number of real roots of $P$ and $\lambda P+(1-\lambda)Q$ are equal.	S
5	Suppose $a,b,c \in \mathbb R^+$ and $\frac1{a^2+1}+\frac1{b^2+1}+\frac1{c^2+1}=2$ Prove that $ab+ac+bc\leq \frac32$	S
6	Suppose $A\in \mathbb R^m$ is closed and non-empty . $f:A\mapsto A$ is a lipchitz function with constant less than 1. (ie there exist $c<1$ that $\|f(x)-f(y)\|<\|x-y\|,\ \forall x,y \in A)$ . Prove that $f$ has a unique point like $x$ that $f(x)=x$	S

Day 2

1	From each vertex of triangle $ABC$ we draw 3 arbitary parrallell lines, and from each vertex we draw a perpendicular to these lines. There are 3 rectangles that one of their diagnals is triangle's side. We draw their other diagnals and call them $\ell_1$ , $\ell_2$ and $\ell_3$ . a) Prove that $\ell_1$ , $\ell_2$ and $\ell_3$ are concurrent at a point $P$ . b) Find the locus of $P$ as we move the 3 arbitary lines.	S
2	Suppose $O$ is circumcenter of triangle $ABC$ . Suppose $\frac{S(OAB)+S(OAC)}2=S(OBC)$ . Prove that ditsance of $O$ (circumcenter) from radial axis of circumcircle and 9-point circle is $\frac {a^2}{\sqrt{9R^2-(a^2+b^2+c^2)}}$	S
3	Prove that in acute-angled traingle ABC if $r$ is inradius and $R$ is radius of circumcircle then: $a^2+b^2+c^2\geq 4(R+r)^2$	S
4	Suppose in triangle $ABC$ incircle touches the side $BC$ at $P$ and $\angle APB=\alpha$ . Prove that : $\frac1{p-b}+\frac1{p-c}=\frac2{rtg\alpha}$	S
5	Suppose $H$ and $O$ are orthocenter and circumcenter of triangle $ABC$ . $\omega$ is circumcircle of $ABC$ . $AO$ intersects with $\omega$ at $A_1$ . $A_1H$ intersects with $\omega$ at $A'$ and $A''$ is the intersection point of $\omega$ and $AH$ . We define points $B',\ B'',\ C'$ and $C''$ similiarly. Prove that $A'A'',B'B''$ and $C'C''$ are concurrent in a point on the Euler line of triangle $ABC$ .	S

Day 3

1	Find all $n,p,q\in \mathbb N$ that:	S
2	$a\in\mathbb N$ and $m=a^2+a+1$ . Find the teh number of $0\leq x\leq m$ that: $x^3\equiv1(\mbox{mod}\ m)$	S
3	$p(x)$ is an irreducible polynomial in $\mathbb Q[x]$ that $\mbox{deg}\ p$ is odd. $q(x),r(x)$ are polynomials with rational coefficients that $p(x)\|q(x)^2+q(x).r(x)+r(x)^2$ . Prove that	S
4	$k$ is an integer. We define the sequence $\{a_n\}_{n=0}^{\infty}$ like this: $a_0=0,\ \ \ a_1=1,\ \ \ a_n=2ka_{n-1}-(k^2+1)a_{n-2}\ \ (n \geq 2)$ $p$ is a prime number that $p\equiv 3(\mbox{mod}\ 4)$ a) Prove that $a_{n+p^2-1}\equiv a_n(\mbox{mod}\ p)$ b) Prove that $a_{n+p^3-p}\equiv a_n(\mbox{mod}\ p^2)$	S
5	$a,b,c\in \matbb N$ that $a,b\neq c$ . Prove that there are infinitely many prime numbers like $P$ that there exist $n\in\mathbb N$ that $p\|a^n+b^n-c^n$	S

Day 4

1	We call the set $A\in \mathbb R^n$ CN if and only if every continuous $f:A\mapsto A$ ther exist : $x\in A$ that $f(x)=x$ a)Example: We know that $A=\{x\in\mathbb R^n\| \|x\|\leq1\}$ is CN b) circle is not CN. Which one of thes sets are CN? 1) A= $\{x\in\mathbb R^3\| \|x\|=1\}\\$ 2) The cross $\{(x,y)\in\mathbb R^2\|xy=0,\ \|x\|+\|y\|\leq1\}$ 3) Graph of $f:[0,1]:\mapsto\mathbb R$ : $f(x)=sin\frac 1x\ \mbox{if}\ x\neq0,\ f(0)=0$	S
2	$n$ vectors are on the plane. We can move each vector forward and backeard on the line that the vector is on it. If there are 2 vectors that their endpoints concide we can omit them and replace them with their sum (If their sum is nonzero). Suppose with these operations with 2 different method we reach to a vector. Prove that these vectors are on a common line	S
3	$f(n)$ is the least number that there exist a $f(n)-$ mino that contains every $n-$ mino. Prove that $10000\leq f(1384)\leq960000$ . Find some bound for $f(n)$	S
4	a) Year 1872 Texas 3 gold miners found a peice of gold. They have a coin that with possibility of $\frac 12$ it will come each side, and they want to give the piece of gold to one of themselves depending on how the coin will come. Design a fair method (It means that each of the 3 miners will win the piece of gold with possibility of $\frac 13$ ) for the miners. b) Year 2005, faculty of Mathematics, Sharif university of Technolgy Suppose $0<\alpha<1$ and we want to find a way for people name $A$ and $B$ that the possibity of winning of $A$ is $\alpha$ . Is it possible to find this way? c) Year 2005 Ahvaz, Takhti Stadium Two soccer teams have a contest. And we want to choose each player's side with the coin, But we don't know that our coin is fair or not. Find a way to find that coin is fair or not? d) Year 2005,summer In the National mathematical Oympiad in Iran. Each student has a coin and must find a way that the possibility of coin being TAIL is $\alpha$ or no. Find a way for the student.	S

Day 5

1	An airplane wants to go from a point on the equator, and at each moment it will go to the northeast with speed $v$ . Suppose the radius of earth is $R$ . a) Will the airplane reach to the north pole? If yes how long it will take to reach the north pole? b) Will the airplne rotate finitely many times around the north pole? If yes how many times?	S
2	We define a relation between subsets of $\mathbb R ^n$ . $A \sim B\Longleftrightarrow$ we can partition $A,B$ in sets $A_1,\dots,A_n$ and $B_1,\dots,B_n$ (i.e $\displaystyle A=\bigcup_{i=1} ^n A_i,\ B=\bigcup_{i=1} ^n B_i, A_i\cap A_j=\emptyset,\ B_i\cap B_j=\emptyset$ ) and $A_i\simeq B_i$ . Say the the following sets have the relation $\sim$ or not ? a) Natural numbers and composite numbers. b) Rational numbers and rational numbers with finite digits in base 10. c) $\{x\in\mathbb Q\|x<\sqrt 2\}$ and $\{x\in\mathbb Q\|x<\sqrt 3\}$ d) $A=\{(x,y)\in\mathbb R^2\|x^2+y^2<1\}$ and $A\setminus \{(0,0)\}$	S
3	For each $m\in \mathbb N$ we define $rad\ (m)=\prod p_i$ that $m=\prod p_i^{\alpha_i}.$ abc Conjecture Suppose $\epsilon >0$ is an arbitary number, then there exist $K$ depinding on $\epsilon$ that for each 3 numbers $a,b,c\in\mathbb Z$ that $gcd (a,b)=1$ and $a+b=c$ then: $max\{\|a\|,\|b\|,\|c\|\}\leq K(rad\ (abc))^{1+\epsilon}$ Now prove each of the foollowing statements with $abc$ conjectures : a) Fermat's last theorem for $n>N$ that $N$ is a natural number. b)We call $n=\prod p_i^{\alpha_i}$ strong if and only $\alpha_i\geq2$ c) Prove that there are finitely many $n$ that $n,\ n+1,\ n+2$ are strong. d) Prove that there are finitely many rational numbers like $\frac pq$ that: $\Big\| \sqrt[3]{2}-\frac pq \Big\|<\frac{2^ {1384}}{q^3}$	S
4	Suppose we have some proteins that each protein is a sequence of 7 "AMINO-ACIDS" $A,\ B,\ C,\ H,\ F,\ N$ . For example $AFHNNNHAFFC$ is a protein. There are some steps that in each step an amino-acid will change to another one. For example with the step $NA\rightarrow N$ the protein $BANANA$ will cahnge to $BANNA$ ("in Persian means workman"). We have a set of allowed steps that each protein can change with these steps. For example with the set of steps: $\\ 1)\ AA\longrightarrow A\\ 2)\ AB\longrightarrow BA\\ 3)\ A\longrightarrow \mbox{null}$ Protein $ABBAABA$ will change like this: $\\ ABB\underline{AA}BA\\ \underline{AB}BABA\\ B\underline{AB}ABA\\ BB\underline{AA}BA\\ BB\underline{AB}A\\ BBB\underline{AA}...$ You see after finite steps this protein will finish it steps. Set of allowed steps that for them there exist a protein that may have infinitely many steps is dangerous. Which of the following allowed sets are dangerous? a) $NO\longrightarrow OONN$ b) $\left\{\begin{array}{c}HHCC\longrightarrow HCCH\\ CC\longrightarrow CH\end{array}\right.$ c) Design a set of allowed steps that change $\underbrace{AA\dots A}_{n}\longrightarrow\underbrace{BB\dots B}_{2^{n}}$ d) Design a set of allowed steps that change $\underbrace{A\dots A}_{n}\underbrace{B\dots B}_{m}\longrightarrow\underbrace{CC\dots C}_{mn}$ You see from $c$ and $d$ that we acn calculate the functions $F(n)=2^{n}$ and $G(M,N)=mn$ with these steps. Find some other calculatable functions with these steps. (It has some extra mark.)	S

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