Olympiad problems

2017 AMC 12/AHSME, 6

Tags: counting

Joy has $30$ thin rods, one each of every integer length from $1$ cm through $30$ cm. She places the rods with lengths $3$ cm, $7$ cm, and $15$ cm on a table. She then wants to choose a fourth rod that she can put with these three to form a quadrilateral with positive area. How many of the remaining rods can she choose as the fourth rod? $\textbf{(A) }16\qquad\textbf{(B) }17\qquad\textbf{(C) }18\qquad\textbf{(D) }19\qquad\textbf{(E) }20$

2014 Contests, 3

Tags: geometry , circumcircle , quadrilateral , tangent , tangent circle

Convex quadrilateral $ABCD$ has $\angle ABC = \angle CDA = 90^{\circ}$. Point $H$ is the foot of the perpendicular from $A$ to $BD$. Points $S$ and $T$ lie on sides $AB$ and $AD$, respectively, such that $H$ lies inside triangle $SCT$ and \[ \angle CHS - \angle CSB = 90^{\circ}, \quad \angle THC - \angle DTC = 90^{\circ}. \] Prove that line $BD$ is tangent to the circumcircle of triangle $TSH$.

1967 Miklós Schweitzer, 5

Tags: function , limit , integration , inequalities , geometry , 3d geometry , logarithm

Let $ f$ be a continuous function on the unit interval $ [0,1]$. Show that \[ \lim_{n \rightarrow \infty} \int_0^1... \int_0^1f(\frac{x_1+...+x_n}{n})dx_1...dx_n=f(\frac12)\] and \[ \lim_{n \rightarrow \infty} \int_0^1... \int_0^1f (\sqrt[n]{x_1...x_n})dx_1...dx_n=f(\frac1e).\]

2013 Stanford Mathematics Tournament, 2

Tags: geometry , analytic geometry

In unit square $ABCD$, diagonals $\overline{AC}$ and $\overline{BD}$ intersect at $E$. Let $M$ be the midpoint of $\overline{CD}$, with $\overline{AM}$ intersecting $\overline{BD}$ at $F$ and $\overline{BM}$ intersecting $\overline{AC}$ at $G$. Find the area of quadrilateral $MFEG$.

2024 Francophone Mathematical Olympiad, 3

Tags: geometry

Let $ABC$ be an acute triangle with $AB<AC$ and let $O$ be its circumcenter. Let $D$ be a point on the segment $AC$ such that $AB=AD$. Let $E$ be the intersection of the line $AB$ with the perpendicular line to $AO$ through $D$. Let $F$ be the intersection of the perpendicular line to $OC$ through $C$ with the line parallel to $AC$ and passing through $E$. Finally, let the lines $CE$ and $DF$ intersect in $G$. Show that $AG$ and $BF$ are parallel.

2020 Online Math Open Problems, 25

Tags:

Let $\mathcal{S}$ denote the set of positive integer sequences (with at least two terms) whose terms sum to $2019$. For a sequence of positive integers $a_1, a_2, \dots, a_k$, its \emph{value} is defined to be \[V(a_1, a_2, \dots, a_k) = \frac{a_1^{a_2} a_2^{a_3} \cdots a_{k-1}^{a_k}}{a_1! a_2! \cdots a_k!}.\] Then the sum of the values over all sequences in $\mathcal{S}$ is $\frac{m}{n}$ where $m$ and $n$ are relatively prime positive integers. Compute the remainder when $m+n$ is divided by $1000$. [i]Proposed by Sean Li[/i]

2011 Gheorghe Vranceanu, 1

Tags: algebra , number theory

If $ \sqrt{x^2+2y+1} +\sqrt[3]{y^3+3x^2+3x+1} $ is rational, then $ x=y. $

2025 Poland - First Round, 9

Tags: number theory

Positive integers $m, n$ are given such that $\sqrt{2}<\frac{m}{n}<\sqrt{2}+\frac{1}{2}$ and $m$ is even. Prove that there exist positive integers $k<m$ and $l<n$ such that $$|\frac{k}{l}-\sqrt{2}|<\frac{m}{n}-\sqrt{2}$$

2003 Moldova Team Selection Test, 3

Tags: geometry , incenter , angle bisector

The sides $ [AB]$ and $ [AC]$ of the triangle $ ABC$ are tangent to the incircle with center $ I$ of the $ \triangle ABC$ at the points $ M$ and $ N$, respectively. The internal bisectors of the $ \triangle ABC$ drawn form $ B$ and $ C$ intersect the line $ MN$ at the points $ P$ and $ Q$, respectively. Suppose that $ F$ is the intersection point of the lines $ CP$ and $ BQ$. Prove that $ FI\perp BC$.

MathLinks Contest 1st, 3

Tags: number theory

Consider $(f_n)_{n\ge 0}$ the Fibonacci sequence, defined by $f_0 = 0$, $f_1 = 1$, $f_{n+1} = f_n + f_{n-1}$ for all positive integers $n$. Solve the following equation in positive integers $$nf_nf_{n+1} = (f_{n+2} - 1)^2.$$ .

2016 JBMO TST - Turkey, 5

Tags: perpendicular lines , geometry

In an acute triangle $ABC$, the feet of the perpendiculars from $A$ and $C$ to the opposite sides are $D$ and $E$, respectively. The line passing through $E$ and parallel to $BC$ intersects $AC$ at $F$, the line passing through $D$ and parallel to $AB$ intersects $AC$ at $G$. The feet of the perpendiculars from $F$ to $DG$ and $GE$ are $K$ and $L$, respectively. $KL$ intersects $ED$ at $M$. Prove that $FM \perp ED$.

2020 Tuymaada Olympiad, 5

Tags: parabola , construction , geometry , algebra , analytic geometry , quadratic , conic

Coordinate axes (without any marks, with the same scale) and the graph of a quadratic trinomial $y = x^2 + ax + b$ are drawn in the plane. The numbers $a$ and $b$ are not known. How to draw a unit segment using only ruler and compass?

1978 IMO Longlists, 44

Tags: trigonometry , inequalities

In $ABC$ with $\angle C = 60^{\circ}$, prove that \[\frac{c}{a} + \frac{c}{b} \ge2.\]

2011 Spain Mathematical Olympiad, 1

Tags: combinatorics

Each pair of vertices of a regular $67$-gon is joined by a line segment. Suppose $n$ of these segments are selected, and each of them is painted one of ten available colors. Find the minimum possible value of $n$ for which, regardless of which $n$ segments were selected and how they were painted, there will always be a vertex of the polygon that belongs to seven segments of the same color.

2001 Slovenia National Olympiad, Problem 3

Tags: geometry

For an arbitrary point $P$ on a given segment $AB$, two isosceles right triangles $APQ$ and $PBR$ with the right angles at $Q$ and $R$ are constructed on the same side of the line $AB$. Prove that the distance from the midpoint $M$ of $QR$ to the line $AB$ does not depend on the choice of $P$.

VI Soros Olympiad 1999 - 2000 (Russia), 9.7

Tags: ratio , geometry

Points $A, B, C$ and $D$ are located on line $\ell$ so that $\frac{AB}{BC}=\frac{AC}{CD}=\lambda $. A certain circle is tangent to line $\ell$ at point $C$. A line is drawn through $A$ that intersects this circle at points $M$ and $N$ such that the bisector perpendiculars to segments $BM$ and $DN$ intersect at point $Q$ on line $\ell$ . In what ratio does point $Q$ divide segment $AD$?

2011 All-Russian Olympiad, 3

Tags: induction , floor function , combinatorics

A convex 2011-gon is drawn on the board. Peter keeps drawing its diagonals in such a way, that each newly drawn diagonal intersected no more than one of the already drawn diagonals. What is the greatest number of diagonals that Peter can draw?

2010 Contests, 2

Tags: linear algebra , matrix

How many ordered pairs of positive integers $(x,y)$ are there such that $y^2-x^2=2y+7x+4$? $ \textbf{(A)}\ 3 \qquad\textbf{(B)}\ 2 \qquad\textbf{(C)}\ 1 \qquad\textbf{(D)}\ 0 \qquad\textbf{(E)}\ \text{Infinitely many} $

Ukrainian From Tasks to Tasks - geometry, 2015.5

Tags: geometry , analytic geometry , coordinate geometry

A coordinate system was constructed on the board, points $A (1,2)$ and B $(3, 1)$ were marked, and then the coordinate system was erased. Restore the coordinate system at the two marked points.

2022 Malaysian IMO Team Selection Test, 6

Tags: geometry

Given a triangle $ABC$ with $AB=AC$ and circumcenter $O$. Let $D$ and $E$ be midpoints of $AC$ and $AB$ respectively, and let $DE$ intersect $AO$ at $F$. Denote $\omega$ to be the circle $(BOE)$. Let $BD$ intersect $\omega$ again at $X$ and let $AX$ intersect $\omega$ again at $Y$. Suppose the line parallel to $AB$ passing through $O$ meets $CY$ at $Z$. Prove that the lines $FX$ and $BZ$ meet at $\omega$. [i]Proposed by Ivan Chan Kai Chin[/i]

2021 Korea Junior Math Olympiad, 3

Tags: geometry , cyclic quadrilateral , circumcircle , perpendicular bisector , tangent

Let $ABCD$ be a cyclic quadrilateral with circumcircle $\Omega$ and let diagonals $AC$ and $BD$ intersect at $X$. Suppose that $AEFB$ is inscribed in a circumcircle of triangle $ABX$ such that $EF$ and $AB$ are parallel. $FX$ meets the circumcircle of triangle $CDX$ again at $G$. Let $EX$ meets $AB$ at $P$, and $XG$ meets $CD$ at $Q$. Denote by $S$ the intersection of the perpendicular bisector of $\overline{EG}$ and $\Omega$ such that $S$ is closer to $A$ than $B$. Prove that line through $S$ parallel to $PQ$ is tangent to $\Omega$.

2017 CCA Math Bonanza, L4.2

Tags:

Find $\arctan\left(1\right)+\arctan\left(2\right)+\arctan\left(3\right)$ in radians. [i]2017 CCA Math Bonanza Lightning Round #4.2[/i]

2017 Romania Team Selection Test, P3

Tags: geometry

Let $ABCD$ be a convex quadrilateral with $\angle ABC = \angle ADC < 90^{\circ}$. The internal angle bisectors of $\angle ABC$ and $\angle ADC$ meet $AC$ at $E$ and $F$ respectively, and meet each other at point $P$. Let $M$ be the midpoint of $AC$ and let $\omega$ be the circumcircle of triangle $BPD$. Segments $BM$ and $DM$ intersect $\omega$ again at $X$ and $Y$ respectively. Denote by $Q$ the intersection point of lines $XE$ and $YF$. Prove that $PQ \perp AC$.

2018 Pan-African Shortlist, A4

Tags: inequalities , algebra

Let $a$, $b$, $c$ and $d$ be non-zero pairwise different real numbers such that $$ \frac{a}{b} + \frac{b}{c} + \frac{c}{d} + \frac{d}{a} = 4 \text{ and } ac = bd. $$ Show that $$ \frac{a}{c} + \frac{b}{d} + \frac{c}{a} + \frac{d}{b} \leq -12 $$ and that $-12$ is the maximum.

1984 IMO Longlists, 67

Tags: geometry , circumcircle , inequalities

With the medians of an acute-angled triangle another triangle is constructed. If $R$ and $R_m$ are the radii of the circles circumscribed about the first and the second triangle, respectively, prove that \[R_m>\frac{5}{6}R\]