2023 IMO Problems

Problems of the 2023 IMO.

Day I

Problem 1

Determine all composite integers $n>1$ that satisfy the following property: if $d_1,d_2,\dots,d_k$ are all the positive divisors of $n$ with $1=d_1<d_2<\dots<d_k=n$ , then $d_i$ divides $d_{i+1}+d_{i+2}$ for every $1\le i \le k-2$ .

Solution

Problem 2

Let $ABC$ be an acute-angled triangle with $AB < AC$ . Let $\Omega$ be the circumcircle of $ABC$ . Let $S$ be the midpoint of the arc $CB$ of $\Omega$ containing $A$ . The perpendicular from $A$ to $BC$ meets $BS$ at $D$ and meets $\Omega$ again at $E \neq A$ . The line through $D$ parallel to $BC$ meets line $BE$ at $L$ . Denote the circumcircle of triangle $BDL$ by $\omega$ . Let $\omega$ meet $\Omega$ again at $P \neq B$ . Prove that the line tangent to $\omega$ at $P$ meets line $BS$ on the internal angle bisector of $\angle BAC$ .

Solution

Problem 3

For each integer $k \geqslant 2$ , determine all infinite sequences of positive integers $a_1, a_2, \ldots$ for which there exists a polynomial $P$ of the form $P(x)=x^k+c_{k-1} x^{k-1}+\cdots+c_1 x+c_0$ , where $c_0, c_1, \ldots, c_{k-1}$ are non-negative integers, such that $P\left(a_n\right)=a_{n+1} a_{n+2} \cdots a_{n+k}$ for every integer $n \geqslant 1$ .

Solution

Day II

Problem 4

Let $x_1, x_2, \cdots , x_{2023}$ be pairwise different positive real numbers such that $a_n = \sqrt{(x_1+x_2+ \text{···} +x_n)(\frac1{x_1} + \frac1{x_2} + \text{···} +\frac1{x_n})}$ is an integer for every $n = 1,2,\cdots,2023$ . Prove that $a_{2023} \ge 3034$ .

Solution

Problem 5

Let $n$ be a positive integer. A Japanese triangle consists of $1 + 2 + \dots + n$ circles arranged in an equilateral triangular shape such that for each $i = 1$ , $2$ , $\dots$ , $n$ , the $i^{th}$ row contains exactly $i$ circles, exactly one of which is coloured red. A ninja path in a Japanese triangle is a sequence of $n$ circles obtained by starting in the top row, then repeatedly going from a circle to one of the two circles immediately below it and finishing in the bottom row. Here is an example of a Japanese triangle with $n = 6$ , along with a ninja path in that triangle containing two red circles.

(image to be inserted)

In terms of $n$ , find the greatest $k$ such that in each Japanese triangle there is a ninja path containing at least $k$ red circles.

Solution

Problem 6

Let $ABC$ be an equilateral triangle. Let $A_1,B_1,C_1$ be interior points of $ABC$ such that $BA_1=A_1C$ , $CB_1=B_1A$ , $AC_1=C_1B$ , and $\angle BA_1C+\angle CB_1A+\angle AC_1B=480^\circ$ Let $BC_1$ and $CB_1$ meet at $A_2,$ let $CA_1$ and $AC_1$ meet at $B_2,$ and let $AB_1$ and $BA_1$ meet at $C_2.$

Prove that if triangle $A_1B_1C_1$ is scalene, then the three circumcircles of triangles $AA_1A_2, BB_1B_2$ and $CC_1C_2$ all pass through two common points.

Solution

2023 IMO (Problems) • Resources
Preceded by 2022 IMO	1 • 2 • 3 • 4 • 5 • 6	Followed by 2024 IMO
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2023 IMO Problems

Contents

Day I

Problem 1

Problem 2

Problem 3

Day II

Problem 4

Problem 5

Problem 6

See Also