Let $D$ be a point in the interior of $\bigtriangleup ABC$ such that $AB = ab$, $AC = ac$, $AD = ad$, $BC = bc$, $BD = bd$ and $CD = cd$. Prove that $\angle ABD + \angle ACD = \frac{\pi}{3}$.

Let $0 < a, b, c < 1$ with $ab + bc + ca = 1$. Prove that \[\frac{a}{1-a^2} + \frac{b}{1-b^2} + \frac{c}{1-c^2} \geq \frac {3 \sqrt{3}}{2}.\] Determine when equality holds.

Let $p \geq 5$ be a prime number. Prove that there exist at least 2 distinct primes $q_1, q_2$ satisfying $1 < q_i < p - 1$ and $q_i^{p-1} \not\equiv 1 \mbox{ (mod }p^2)$, for $i = 1, 2$.

Let $x_0, x_1, x_2, \ldots$ be the sequence defined by $x_i= 2^i$ if $0 \leq i \leq 2003$ $x_i=\sum_{j=1}^{2004} x_{i-j}$ if $i \geq 2004$ Find the greatest $k$ for which the sequence contains $k$ consecutive terms divisible by 2004.

Let $ABC$ be an isosceles triangle with $AC=BC$, whose incentre is $I$. Let $P$ be a point on the circumcircle of the triangle $AIB$ lying inside the triangle $ABC$. The lines through $P$ parallel to $CA$ and $CB$ meet $AB$ at $D$ and $E$, respectively. The line through $P$ parallel to $AB$ meets $CA$ and $CB$ at $F$ and $G$, respectively. Prove that the lines $DF$ and $EG$ intersect on the circumcircle of the triangle $ABC$. Proposed by Hojoo Lee, Korea

Find all functions $ f: \mathbb{R} \to \mathbb{R}$ satisfying \[ f\left(\frac {x + y}{x - y}\right) = \frac {f\left(x\right) + f\left(y\right)}{f\left(x\right) - f\left(y\right)} \] for all $ x \neq y$.