OMA 2020

For every positive integer $n$ , let $S (n)$ be the sum of the digits of $n$ . Find, if any, a $171$ -digit positive integer $n$ such that $7$ divides $S (n)$ and $7$ divides $S (n + 1)$ .

Let $k\ge 1$ be an integer. Determine the smallest positive integer $n$ such that some cells on an $n \times n$ board can be painted black so that in each row and in each column there are exactly $k$ black cells, and furthermore, the black cells do not share a side or a vertex with another black square.

Clarification: You have to answer n based on $k$ .

Let $ABC$ be a right isosceles triangle with right angle at $A$ . Let $E$ and $F$ be points on A $B$ and $AC$ respectively such that $\angle ECB = 30^o$ and $\angle FBC = 15^o$ . Lines $CE$ and $BF$ intersect at $P$ and line $AP$ intersects side $BC$ at $D$ . Calculate the measure of angle $\angle FDC$ .

Let $a$ and $b$ be positive integers such that $\frac{5a^4 + a^2}{b^4 + 3b^2 + 4}$ is an integer. Show that $a$ is not prime.

Determine the highest possible value of: $$S = a_1a_2a_3 + a_4a_5a_6 +... + a_{2017}a_{2018}a_{2019} + a_{2020}$$ where $(a_1, a_2, a_3,..., a_{2020})$ is a permutation of $(1,2,3,..., 2020)$ .

Clarification: In $S$ , each term, except the last one, is the multiplication of three numbers.

Let $n\ge 3$ be an integer. Lucas and Matías play a game in a regular $n$ -sided polygon with a vertex marked as a trap. Initially Matías places a token at one vertex of the polygon. In each step, Lucas says a positive integer and Matías moves the token that number of vertices clockwise or counterclockwise, at his choice.

a) Determine all the $n\ge 3$ such that Matías can locate the token and move it in such a way as to never fall into the trap, regardless of the numbers Lucas says. Give the strategy to Matías.

b) Determine all the $n\ge 3$ such that Lucas can force Matías to fall into the trap. Give the strategy to Lucas.

Note. The two players know the value of $n$ and see the polygon.