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At what value of $k$ is the quantity $A_k$ = $(19^k + 66^k)/k!$ at its maximum? You are given a number x that is greater than 1. Is the following inequality necessarily fulfilled $[\sqrt{\sqrt{x}}] = [\sqrt{\sqrt{x}}]$?

Prove that for any positive integer n, it is always possible to find a number, consisting of digits 1 and 2, that is divisible by $2^n$.

$($For example, 2 divides by 2, 12 divides by 4, 112 divides by 8, 2112 divides by 16 and so on …$)$.

Author: G. Zhukov

The square trinomial $f (x) = ax^2 + bx + c$ that does not have roots is such that the coefficient b is rational, and among the numbers c and $f (c)$ there is exactly one irrational.

Can the discriminant of the trinomial $f (x)$ be rational?

The bus has n seats, and all of the tickets are sold to n passengers. The first to enter the bus is the Scattered Scientist and, without looking at his ticket, takes a random available seat. Following this, the passengers enter one by one. If the new passenger sees that his place is free, he takes his place. If the place is occupied, then the person who gets on the bus takes the first available seat. Find the probability that the passenger who got on the bus last will take his seat according to his ticket?

Author: I.I. Bogdanov

Peter wants to write down all of the possible sequences of 100 natural numbers, in each of which there is at least one 4 or 5, and any two neighbouring terms differ by no more than 2. How many sequences will he have to write out?

Author: I.I. Bogdanov

Peter wants to write down all of the possible sequences of 100 natural numbers, in each of which there is at least one 3, and any two neighbouring terms differ by no more than 1. How many sequences will he have to write out?

A function f is given, defined on the set of real numbers and taking real values. It is known that for any x and y such that x $>$ y, the inequality $(f (x))$ ² $≤$ f $(y)$ is true. Prove that the set of values generated by the function is contained in the interval [0,1].

There are 8 glasses of water on the table. You are allowed to take any two of the glasses and make them have equal volumes of water (by pouring some water from one glass into the other). Prove that, by using such operations, you can eventually get all the glasses to contain equal volumes of water.

Prove that if the irreducible rational fraction p/q is a root of the polynomial $P (x)$ with integer coefficients, then $P (x) = (qx – p) Q (x)$, where the polynomial $Q (x)$ also has integer coefficients.

We are given $n+1$ different natural numbers, which are less than $2n$ $(n>1)$. Prove that among them there will always be three numbers, where the sum of two of them is equal to the third.

On a particular day it turned out that every person living in a particular city made no more than one phone call. Prove that it is possible to divide the population of this city into no more than three groups, so that within each group no person spoke to any other by telephone.

With a non-zero number, the following operations are allowed: $x \rightarrow \frac{1+x}{x}, x \rightarrow \frac{1-x}{x}$. Is it true that from every non-zero rational number one can obtain each rational number with the help of a finite number of such operations?