$f$ is a function twice differentiable on $[0,1]$ and such that $f''$ is continuous. We suppose that : $f(1)-1=f(0)=f'(1)=f'(0)=0$. Prove that there exists $x_0$ on $[0,1]$ such that $|f''(x_0)| \geq 4$

Let $a$, $b$, $c$ be positive real numbers with $abc \leq a+b+c$. Show that $$a^2 + b^2 + c^2 \geq \sqrt 3 abc.$$ Cristinel Mortici, Romania

Any rational number admits a non-decimal representation unlimited decimal expansion. This development has the particularity of being periodic. Examples: $\frac{1}{7} = 0.142857142857…$ has a period $6$ while $\frac{1}{11}=0.0909090909 …$ $2$ periodic. What are the reciprocals of the prime integers with a period less than or equal to five?

Find all triangles whose side lengths are consecutive integers, and one of whose angles is twice another.

In a sports meeting a total of $m$ medals were awarded over $n$ days. On the first day one medal and $\frac{1}{7}$ of the remaining medals were awarded. On the second day two medals and $\frac{1}{7}$ of the remaining medals were awarded, and so on. On the last day, the remaining $n$ medals were awarded. How many medals did the meeting last, and what was the total number of medals ?

Find the integer represented by $\left[ \sum_{n=1}^{10^9} n^{-2/3} \right]$. Here $[x]$ denotes the greatest integer less than or equal to $x.$

Let $G$ be a non-empty set of non-constant functions $f$ such that $f(x)=ax + b$ (where $a$ and $b$ are two reals) and satisfying the following conditions: 1) if $f \in G$ and $g \in G$ then $gof \in G$, 2) if $f \in G$ then $f^ {-1} \in G$, 3) for all $f \in G$ there exists $x_f \in \mathbb{R}$ such that $f(x_f)=x_f$. Prove that there is a real $k$ such that for all $f \in G$ we have $f(k)=k$

Let $ABCDE$ be a convex pentagon such that $$\angle BAC = \angle CAD = \angle DAE\qquad \text{and}\qquad \angle ABC = \angle ACD = \angle ADE.$$ The diagonals $BD$ and $CE$ meet at $P$. Prove that the line $AP$ bisects the side $CD$. Proposed by Zuming Feng, USA