Found problems: 85335
2021 MIG, 3
$20\%$ of $10$ is exactly $5\%$ of what number?
$\textbf{(A) }20\qquad\textbf{(B) }30\qquad\textbf{(C) }40\qquad\textbf{(D) }50\qquad\textbf{(E) }60$
2019 Purple Comet Problems, 30
A [i]derangement [/i] of the letters $ABCDEF$ is a permutation of these letters so that no letter ends up in the position it began such as $BDECFA$. An [i]inversion [/i] in a permutation is a pair of letters $xy$ where $x$ appears before $y$ in the original order of the letters, but $y$ appears before $x$ in the permutation. For example, the derangement $BDECFA$ has seven inversions: $AB, AC, AD, AE, AF, CD$, and $CE$. Find the total number of inversions that appear in all the derangements of $ABCDEF$.
PEN G Problems, 9
Show that $\cos \frac{\pi}{7}$ is irrational.
2014 BMT Spring, 12
A two-digit integer is [i]reversible [/i] if, when written backwards in base $10$, it has the same number of positive divisors. Find the number of reversible integers.
2022 Iran Team Selection Test, 8
In triangle $ABC$, with $AB<AC$, $I$ is the incenter, $E$ is the intersection of $A$-excircle and $BC$. Point $F$ lies on the external angle bisector of $BAC$ such that $E$ and $F$ lieas on the same side of the line $AI$ and $\angle AIF=\angle AEB$. Point $Q$ lies on $BC$ such that $\angle AIQ=90$. Circle $\omega_b$ is tangent to $FQ$ and $AB$ at $B$, circle $\omega_c$ is tangent to $FQ$ and $AC$ at $C$ and both circles pass through the inside of triangle $ABC$. if $M$ is the Midpoint od the arc $BC$, which does not contain $A$, prove that $M$ lies on the radical axis of $\omega_b$ and $\omega_c$.
Proposed by Amirmahdi Mohseni
2005 Colombia Team Selection Test, 2
The following operation is allowed on a finite graph: Choose an arbitrary cycle of length 4 (if there is any), choose an arbitrary edge in that cycle, and delete it from the graph. For a fixed integer ${n\ge 4}$, find the least number of edges of a graph that can be obtained by repeated applications of this operation from the complete graph on $n$ vertices (where each pair of vertices are joined by an edge).
[i]Proposed by Norman Do, Australia[/i]
1998 National High School Mathematics League, 7
$f(x)$ is an even function with period of $2$. If $f(x)=x^{\frac{1}{1000}}$ when $x\in[0,1]$, then the order of $f\left(\frac{98}{19}\right),f\left(\frac{101}{17}\right),f\left(\frac{104}{15}\right)$ is________(from small to large).
2013 Oral Moscow Geometry Olympiad, 2
With a compass and a ruler, split a triangle into two smaller triangles with the same sum of squares of sides.
Gheorghe Țițeica 2024, P4
For a set $M$ of $n\geq 3$ points in the plane we define a [i]path[/i] to be a polyline $A_1A_2\dots A_n$, where $M=\{A_1,A_2,\dots ,A_n\}$ and define its length to be $A_1A_2+A_2A_3+\dots +A_{n-1}A_n$. We call $M$ [i]path-unique[/i] if any two distinct paths have different lengths and [i]segment-unique[/i] if any two nondegenerate segments with their ends among the points in $M$ have different lengths. Determine the positive integers $n\geq 3$ such that:
a) any segment-unique set $M$ of $n$ points in the plane is path-unique;
b) any path-unique set $M$ of $n$ points in the plane is segment-unique.
[i]Cristi Săvescu[/i]
2007 AMC 8, 3
What is the sum of the two smallest prime factors of $250$?
$\textbf{(A)}\ 2 \qquad
\textbf{(B)}\ 5 \qquad
\textbf{(C)}\ 7 \qquad
\textbf{(D)}\ 10 \qquad
\textbf{(E)}\ 12$
2017 Morocco TST-, 6
For any positive integer $k$, denote the sum of digits of $k$ in its decimal representation by $S(k)$. Find all polynomials $P(x)$ with integer coefficients such that for any positive integer $n \geq 2016$, the integer $P(n)$ is positive and $$S(P(n)) = P(S(n)).$$
[i]Proposed by Warut Suksompong, Thailand[/i]
1962 All-Soviet Union Olympiad, 7
Let $a;b;c;d>0$ such that $abcd=1$. Prove that $a^2+b^2+c^2+d^2+a(b+c)+b(c+d)+c(d+a)\ge 10$
2012 Princeton University Math Competition, B2
A $6$-inch-wide rectangle is rotated $90$ degrees about one of its corners, sweeping out an area of $45\pi$ square inches, excluding the area enclosed by the rectangle in its starting position. Find the rectangle’s length in inches.
2012 Princeton University Math Competition, A5
What is the smallest natural number $n$ greater than $2012$ such that the polynomial $f(x) =(x^6 + x^4)^n - x^{4n} - x^6$ is divisible by $g(x) = x^4 + x^2 + 1$?
1977 Canada National Olympiad, 3
$N$ is an integer whose representation in base $b$ is $777$. Find the smallest integer $b$ for which $N$ is the fourth power of an integer.
1998 AMC 12/AHSME, 10
A large square is divided into a small square surrounded by four congruent rectangles as shown. The perimeter of each of the congruent rectangles is 14. What is the area of the large square?
[asy]unitsize(3mm);
defaultpen(linewidth(.8pt));
draw((0,0)--(7,0)--(7,7)--(0,7)--cycle);
draw((1,0)--(1,6));
draw((7,1)--(1,1));
draw((6,7)--(6,1));
draw((0,6)--(6,6));[/asy]$ \textbf{(A)}\ \ 49 \qquad \textbf{(B)}\ \ 64 \qquad \textbf{(C)}\ \ 100 \qquad \textbf{(D)}\ \ 121 \qquad \textbf{(E)}\ \ 196$
2005 Germany Team Selection Test, 3
Let $a$, $b$, $c$, $d$ and $n$ be positive integers such that $7\cdot 4^n = a^2+b^2+c^2+d^2$. Prove that the numbers $a$, $b$, $c$, $d$ are all $\geq 2^{n-1}$.
2006 All-Russian Olympiad Regional Round, 8.5
The product $a_1 \cdot a_2 \cdot ... \cdot a_{100}$ is written on the board , where $a_1$, $a_2$, $ ... $, $a_{100}$, are natural numbers. Let's consider $99$ expressions, each of which is obtained by replacing one of the multiplication signs with an addition sign. It is known that the values of exactly $32$ of these expressions are even. What is the largest number of even numbers among $a_1$, $a_2$, $ ... $, $a_{100}$ could it be?
2015 India Regional MathematicaI Olympiad, 1
Let ABC be a triangle. Let B' and C' denote the reflection of B and C in the internal angle bisector of angle A. Show that the triangles ABC and AB'C' have the same incenter.
2003 AMC 10, 12
A point $ (x,y)$ is randomly picked from inside the rectangle with vertices $ (0,0)$, $ (4,0)$, $ (4,1)$, and $ (0,1)$. What is the probability that $ x<y$?
$ \textbf{(A)}\ \frac{1}{8} \qquad
\textbf{(B)}\ \frac{1}{4} \qquad
\textbf{(C)}\ \frac{3}{8} \qquad
\textbf{(D)}\ \frac{1}{2} \qquad
\textbf{(E)}\ \frac{3}{4}$
2020 New Zealand MO, 4
Determine all prime numbers $p$ such that $p^2 - 6$ and $p^2 + 6$ are both prime numbers.
2011 VJIMC, Problem 1
Let $n>k$ and let $A_1,\ldots,A_k$ be real $n\times n$ matrices of rank $n-1$. Prove that
$$A_1\cdots A_k\ne0.$$
2017 Polish MO Finals, 4
Prove that the set of positive integers $\mathbb Z^+$ can be represented as a sum of five pairwise distinct subsets with the following property: each $5$-tuple of numbers of form $(n,2n,3n,4n,5n)$, where $n\in\mathbb Z^+$, contains exactly one number from each of these five subsets.
2022 Stars of Mathematics, 4
Let $a{}$ be an even positive integer which is not a power of two. Prove that at least one of $2^{2^n}+1$ and $a^{2^n}+1$ is composite, for infinitely many positive integers $n$.
[i]Bojan Bašić[/i]
2016 CMIMC, 10
Let $\mathcal{P}$ be the unique parabola in the $xy$-plane which is tangent to the $x$-axis at $(5,0)$ and to the $y$-axis at $(0,12)$. We say a line $\ell$ is $\mathcal{P}$-friendly if the $x$-axis, $y$-axis, and $\mathcal{P}$ divide $\ell$ into three segments, each of which has equal length. If the sum of the slopes of all $\mathcal{P}$-friendly lines can be written in the form $-\tfrac mn$ for $m$ and $n$ positive relatively prime integers, find $m+n$.