A solid cube of side length $1$ is removed from each corner of a solid cube of side length $3$. How many edges does the remaining solid have? $\textbf{(A) }36\qquad \textbf{(B) }60\qquad \textbf{(C) }72\qquad \textbf{(D) }84\qquad \textbf{(E) }108\qquad$

Let $p \ge 5$ be a prime number. For a positive integer $k$, let $R(k)$ be the remainder when $k$ is divided by $p$, with $0 \le R(k) \le p-1$. Determine all positive integers $a < p$ such that, for every $m = 1, 2, \cdots, p-1$, $$ m + R(ma) > a. $$

Let $ABCD$ be a trapezium in which $AB //CD$ and $AD \perp AB$. Suppose $ABCD$ has an incircle which touches $AB$ at $Q$ and $CD$ at $P$. Given that $PC = 36$ and $QB = 49$, find $PQ$.

In a given country, all inhabitants are knights or knaves. A knight never lies; a knave always lies. We meet three persons, $A, B$, and $C$. Person $A$ says, “If $C$ is a knight, $B$ is a knave.” Person $C$ says, “$A$ and I are different; one is a knight and the other is a knave.” Who are the knights, and who are the knaves ?

A sequence $(u_{n})$ is defined by \[ u_{0}=2 \quad u_{1}=\frac{5}{2}, u_{n+1}=u_{n}(u_{n-1}^{2}-2)-u_{1} \quad \textnormal{for } n=1,\ldots \] Prove that for any positive integer $n$ we have \[ [u_{n}]=2^{\frac{(2^{n}-(-1)^{n})}{3}} \](where [x] denotes the smallest integer $\leq$ x)$.$

Let $ I $ be the incenter of a triangle $ ABC $ with $ AB \neq AC $, and let $ M $ be the midpoint of the arc $ BAC $ of the circumcircle of the triangle. The perpendicular line to $ AI $ passing through $ I $ intersects line $ BC $ at point $ D $. The line $ MI $ intersects the circumcircle of triangle $ BIC $ at point $ N $. Prove that line $ DN $ is tangent to the circumcircle of triangle $ BIC $.

For given positive integer $a$, find every $(x_1, x_2, …, x_{2002})$ that satisfies the following: (1) $x_1 \geq x_2 \geq … \geq x_{2002} \geq 0$ (2) $0< x_1+x_2+…+x_{2003}<a+1$ (3) $ x^2_1+x^2_2+…+x^2_{2003}+9=a^2$

Values for $A,B,C,$ and $D$ are to be selected from $\{1, 2, 3, 4, 5, 6\}$ without replacement (i.e. no two letters have the same value). How many ways are there to make such choices so that the two curves $y=Ax^2+B$ and $y=Cx^2+D$ intersect? (The order in which the curves are listed does not matter; for example, the choices $A=3, B=2, C=4, D=1$ is considered the same as the choices $A=4, B=1, C=3, D=2.$) $\textbf{(A) }30 \qquad \textbf{(B) }60 \qquad \textbf{(C) }90 \qquad \textbf{(D) }180 \qquad \textbf{(E) }360$

[b]i)[/b] Let $0<a<b$.Prove that amongst all triangles having base $a$ and perimeter $a+b$ the triangle having two sides(other than the base) equal to $\frac {b}{2}$ has the maximum area. [b]ii)[/b]Using $i)$ or otherwise, prove that amongst all quadrilateral having give perimeter the square has the maximum area.

How many ordered pairs of nonnegative integers $\left(a,b\right)$ are there with $a+b=999$ such that each of $a$ and $b$ consists of at most two different digits? (These distinct digits need not be the same digits in both $a$ and $b$. For example, we might have $a=622$ and $b=377$.)

$k_1$ denotes one of the arcs formed by intersection of the circumference $k$ and the chord $AB$. $C$ is the middle point of $k_1$. On the half line (ray) $PC$ is drawn the segment $PM$. Find the locus formed from the point $M$ when $P$ is moving on $k_1$. [i]G. Ganchev[/i]

In order to pass $ B$ going $ 40$ mph on a two-lane highway, $ A$, going $ 50$ mph, must gain $ 30$ feet. Meantime, $ C$, $ 210$ feet from $ A$, is headed toward him at $ 50$ mph. If $ B$ and $ C$ maintain their speeds, then, in order to pass safely, $ A$ must increase his speed by: $ \textbf{(A)}\ \text{30 mph} \qquad \textbf{(B)}\ \text{10 mph} \qquad \textbf{(C)}\ \text{5 mph} \qquad \textbf{(D)}\ \text{15 mph} \qquad \textbf{(E)}\ \text{3 mph}$

Points $A, B, C$ and $D$ are on a circle of diameter $1$, and $X$ is on diameter $\overline{AD}$. If $BX=CX$ and $3 \angle BAC=\angle BXC=36^{\circ}$, then $AX=$ [asy] draw(Circle((0,0),10)); draw((-10,0)--(8,6)--(2,0)--(8,-6)--cycle); draw((-10,0)--(10,0)); dot((-10,0)); dot((2,0)); dot((10,0)); dot((8,6)); dot((8,-6)); label("A", (-10,0), W); label("B", (8,6), NE); label("C", (8,-6), SE); label("D", (10,0), E); label("X", (2,0), NW); [/asy] $ \textbf{(A)}\ \cos 6^{\circ}\cos 12^{\circ} \sec 18^{\circ} \qquad\textbf{(B)}\ \cos 6^{\circ}\sin 12^{\circ} \csc 18^{\circ} \qquad\textbf{(C)}\ \cos 6^{\circ}\sin 12^{\circ} \sec 18^{\circ} \\ \qquad\textbf{(D)}\ \sin 6^{\circ}\sin 12^{\circ} \csc 18^{\circ} \qquad\textbf{(E)}\ \sin 6^{\circ} \sin 12^{\circ} \sec 18^{\circ} $

Find the number of positive integers less than $2018$ that are divisible by $6$ but are not divisible by at least one of the numbers $4$ or $9$.

Prove that the set of positive integers cannot be partitioned into three nonempty subsets such that, for any two integers $x,y$ taken from two different subsets, the number $x^2-xy+y^2$ belongs to the third subset.

In $10$ years the product of Melanie's age and Phil's age will be $400$ more than it is now. Find what the sum of Melanie's age and Phil's age will be $6$ years from now.

A road company is trying to build a system of highways in a country with $21$ cities. Each highway runs between two cities. A trip is a sequence of distinct cities $C_1,\dots, C_n$, for which there is a highway between $C_i$ and $C_{i+1}$. The company wants to fulfill the following two constraints: (1) for any ordered pair of distinct cities $(C_i, C_j)$, there is exactly one trip starting at $C_i$ and ending at $C_j$. (2) if $N$ is the number of trips including exactly 5 cities, then $N$ is maximized. What is this maximum value of $N$?

A fixed triangle $ABC$ is right angled at $A$, and $M$ is a fixed point inside the triangle such that $BM=BA$. Let $O$ be a point on line $BC$, and suppose the ray $OM$ beyond $M$ intersects the interior and exterior angle bisector of $\angle ACM$ at $S$ and $T$ respectively. Prove that there exist a fixed point $J$ such that circumcircles of triangles $JOM$ and $CST$ are always tangent, regardless of the choice of $O$. [i]Proposed by Ivan Chan Kai Chin[/i]

$f$ is a differentiable function such that $f(f(x))=x$ where $x \in [0,1]$.Also $f(0)=1$.Find the value of $$\int_0^1(x-f(x))^{2016}dx$$

The schematic wiring diagram below is made of six electric resistances $a_1,a_2,a_3,a_4,a_5,a_6(a_1>a_2>a_3>a_4>a_5>a_6)$. How can we choose the electric resistances, so that the all-in resistance takes its minimum value? [center][img]https://services.artofproblemsolving.com/download.php?id=YXR0YWNobWVudHMvNy81LzBiZGVjZjczN2Y4MWY0YmNhMTg1YmQxNGEzZWMwZDc2OTE1NzUwLnBuZw==&rn=MjExMTExMTExMTExMTFnaGoucG5n[/img][/center]

Compute the sum of all real numbers x which satisfy the following equation $$\frac {8^x - 19 \cdot 4^x}{16 - 25 \cdot 2^x}= 2$$

Is it possible to dissect an equilateral triangle into three congruent polygonal pieces (not necessarily convex), one of which contains the triangle’s centre in its interior? [i]Note:[/i] The interior of a polygon is the polygon without its boundary. [i]Proposed by Dylan Toh[/i]

Let $A_1,A_2,\cdots,A_n$ be $n$ distinct points on the plane ($n>1$). We consider all the segments $A_iA_j$ where $i<j\leq{n}$ and color the midpoints of them. What's the minimum number of colored points? (In fact, if $k$ colored points coincide, we count them $1$.)

Let $a,b,c$ be positive real numbers with $a+b+c=3$. Prove that \[ \sqrt{\frac{b}{a^2+3}}+ \sqrt{\frac{c}{b^2+3}}+ \sqrt{\frac{a}{c^2+3}} ~\le~ \frac32\sqrt[4]{\frac{1}{abc}}\]

Let $n$ be a positive integer. The real numbers $a_1,a_2,\cdots,a_n$ and $r_1,r_2,\cdots,r_n$ are such that $a_1\leq a_2\leq \cdots \leq a_n$ and $0\leq r_1\leq r_2\leq \cdots \leq r_n$. Prove that $\sum_{i=1}^n\sum_{j=1}^n a_i a_j \min (r_i,r_j)\geq 0$