2017 Ecuador NMO (OMEC)

Day 1

1

Determine what day of the week day was: June $6$, $1944$. Note: Leap years are those that are multiples of $4$ and do not end in $00$ or that are multiples of $400$, for example $1812$, $1816$, $1820$, $1600$, $2000$, but $1800$, $1810$, $2100$ are not leaps. Giving the answer without any mathematical justification will not award points.

2

Let $ABC$ be a triangle with $AC = 18$ and $D$ is the point on the segment $AC$ such that $AD = 5$. Draw perpendiculars from $D$ on $AB$ and $BC$ which have lengths $4$ and $5$ respectively. Find the area of the triangle $ABC$.

3

Adrian has $2n$ cards numbered from $ 1$ to $2n$. He gets rid of $n$ cards that are consecutively numbered. The sum of the numbers of the remaining papers is $1615$. Find all the possible values of $n$.

Day 2

4

Sebastian, the traveling ant, walks on top of some square boards. He just walks horizontally or vertically through the squares of the boards and does not pass through the same square twice. On a board of $7\times 7$, in which squares can Sebastian start his journey so that he can pass through all the squares on the board?

5

Let the sequences $(x_n)$ and $(y_n)$ be defined by $x_0 = 0$, $x_1 = 1$, $x_{n + 2} = 3x_{n + 1}-2x_n$ for $n = 0, 1, ...$ and $y_n = x^2_n+2^{n + 2}$ for $n = 0, 1, ...,$ respectively. Show that for all n> 0, and n is the square of a odd integer.

6

Let $ABCDEF$ be a convex hexagon with sides not parallel and tangent to a circle $\Gamma$ at the midpoints $P$, $Q$, $R$ of the sides AB, $CD$, $EF$ respectively. $\Gamma$ is tangent to $BC$, $DE$ and $FA$ at the points $X, Y, Z$ respectively. Line $AB$ intersects lines $EF$ and $CD$ at points $M$ and $N$ respectively. Lines $MZ$ and $NX$ intersect at point $K$. Let $ r$ be the line joining the center of $\Gamma$ and point $K$. Prove that the intersection of $PY$ and $QZ$ lies on the line $ r$.