Olympiad problems

2011 Tournament of Towns, 3

Tags: weighing , combinatorics

A balance and a set of pairwise different weights are given. It is known that for any pair of weights from this set put on the left pan of the balance, one can counterbalance them by one or several of the remaining weights put on the right pan. Find the least possible number of weights in the set.

1990 AMC 12/AHSME, 21

Tags: geometry , 3d geometry , pyramid , trigonometry , pythagorean theorem

Consider a pyramid $P-ABCD$ whose base $ABCD$ is a square and whose vertex $P$ is equidistant from $A$, $B$, $C$, and $D$. If $AB=1$ and $\angle APD=2\theta$ then the volume of the pyramid is $\text{(A)} \ \frac{\sin \theta}{6} \qquad \text{(B)} \ \frac{\cot \theta}{6} \qquad \text{(C)} \ \frac1{6\sin \theta} \qquad \text{(D)} \ \frac{1-\sin 2\theta}{6} \qquad \text{(E)} \ \frac{\sqrt{\cos 2\theta}}{6\sin \theta}$

1999 Putnam, 5

Tags: polynomial , integration , calculus , inequalities , real analysis

Prove that there is a constant $C$ such that, if $p(x)$ is a polynomial of degree $1999$, then \[|p(0)|\leq C\int_{-1}^1|p(x)|\,dx.\]

2010 Austria Beginners' Competition, 3

Tags: function

Let $x$ and $y$ be positive real numbers with $x + y =1 $. Prove that $$\frac{(3x-1)^2}{x}+ \frac{(3y-1)^2}{y} \ge1.$$ For which $x$ and $y$ equality holds? (K. Czakler, GRG 21, Vienna)

2014 Israel National Olympiad, 1

Tags: modular arithmetic , digit , number theory

Consider the number $\left(101^2-100^2\right)\cdot\left(102^2-101^2\right)\cdot\left(103^2-102^2\right)\cdot...\cdot\left(200^2-199^2\right)$. [list=a] [*] Determine its units digit. [*] Determine its tens digit. [/list]

2001 Brazil Team Selection Test, Problem 2

Tags: number theory

Let $f(n)$ denote the least positive integer $k$ such that $1+2+\cdots+k$ is divisible by $n$. Show that $f(n)=2n-1$ if and only if $n$ is a power of $2$.

2017 AMC 10, 6

Tags:

What is the largest number of solid $2\text{-in}\times 2\text{-in}\times 1\text{-in}$ blocks that can fit in a $3\text{-in}\times 2\text{-in}\times 3\text{-in}$ box? $\textbf{(A) } 3\qquad \textbf{(B) } 4\qquad \textbf{(C) } 5\qquad \textbf{(D) } 6\qquad \textbf{(E) } 7$

2004 Manhattan Mathematical Olympiad, 1

Tags: geometry

Suppose two triangles have equal areas and equal perimeters. Prove that, if a side of one triangle is congruent to a side of the other triangle, then the two triangles are congruent.

2008 Thailand Mathematical Olympiad, 4

Tags: inequalities , algebra

Prove that $$\sqrt{a^2 + b^2 -\sqrt2 ab} +\sqrt{b^2 + c^2 -\sqrt2 bc} \ge \sqrt{a^2 + c^2}$$ for all real numbers $a, b, c > 0$

2012 Turkey Team Selection Test, 2

Tags: inequalities , pythagorean theorem , geometry

In a plane, the six different points $A, B, C, A', B', C'$ are given such that triangles $ABC$ and $A'B'C'$ are congruent, i.e. $AB=A'B', BC=B'C', CA=C'A'.$ Let $G$ be the centroid of $ABC$ and $A_1$ be an intersection point of the circle with diameter $AA'$ and the circle with center $A'$ and passing through $G.$ Define $B_1$ and $C_1$ similarly. Prove that \[ AA_1^2+BB_1^2+CC_1^2 \leq AB^2+BC^2+CA^2 \]