CMO 2020

Let $a_1,a_2,\cdots,a_{41}\in\mathbb{R},$ such that $a_{41}=a_1, \sum_{i=1}^{40}a_i=0,$ and for any $i=1,2,\cdots,40, |a_i-a_{i+1}|\leq 1.$ Determine the greatest possible value of $(1)a_{10}+a_{20}+a_{30}+a_{40};$ $(2)a_{10}\cdot a_{20}+a_{30}\cdot a_{40}.$

In triangle $ABC$ , $AB>AC.$ The bisector of $\angle BAC$ meets $BC$ at $D.$ $P$ is on line $DA,$ such that $A$ lies between $P$ and $D$ . $PQ$ is tangent to $\odot(ABD)$ at $Q.$ $PR$ is tangent to $\odot(ACD)$ at $R.$ $CQ$ meets $BR$ at $K.$ The line parallel to $BC$ and passing through $K$ meets $QD,AD,RD$ at $E,L,F,$ respectively. Prove that $EL=KF.$

Let $S$ be a set, $|S|=35$ . A set $F$ of mappings from $S$ to itself is called to be satisfying property $P(k)$ , if for any $x,y\in S$ , there exist $f_1, \cdots, f_k \in F$ (not necessarily different), such that $f_k(f_{k-1}(\cdots (f_1(x))))=f_k(f_{k-1}(\cdots (f_1(y))))$ . Find the least positive integer $m$ , such that if $F$ satisfies property $P(2019)$ , then it also satisfies property $P(m)$ .

Find the largest positive constant $C$ such that the following is satisfied: Given $n$ arcs (containing their endpoints) $A_1,A_2,\ldots ,A_n$ on the circumference of a circle, where among all sets of three arcs $(A_i,A_j,A_k)$ $(1\le i< j< k\le n)$ , at least half of them has $A_i\cap A_j\cap A_k$ nonempty, then there exists $l>Cn$ , such that we can choose $l$ arcs among $A_1,A_2,\ldots ,A_n$ , whose intersection is nonempty.

Given any positive integer $c$ , denote $p(c)$ as the largest prime factor of $c$ . A sequence $\{a_n\}$ of positive integers satisfies $a_1>1$ and $a_{n+1}=a_n+p(a_n)$ for all $n\ge 1$ . Prove that there must exist at least one perfect square in sequence $\{a_n\}$ .

Does there exist positive reals $a_0, a_1,\ldots ,a_{19}$ , such that the polynomial $P(x)=x^{20}+a_{19}x^{19}+\ldots +a_1x+a_0$ does not have any real roots, yet all polynomials formed from swapping any two coefficients $a_i,a_j$ has at least one real root?