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1. Let ABC be a triangle. Let D, E, F be points respectively on the segments BC, CA, AB such that AD, BE, CF concur at the point K. Suppose BD / DC = BF / FA and ∠ADB = ∠AFC. Prove that ∠ ABE = ∠ CAD.
2. Let $(a_1a_2a_3.....a_{2011})$ be a permutation (that is arearrangement) of the numbers 1, 2, 3 . . . , 2011. Show that there exist two numbers j, k. such that $1 \le j < k \le 2011$ and $|a_{j}-j|=|a_{k}-k|$.
3. A natural number n is chosen strictly between two consecutive perfect square. The smaller of these two squares is obtained by subtracting k from n and the larger one is obtained by adding l to n. Prove that $n-kl$ is a perfect square.
4. Consider a 20-sided convex polygon K, with vertice $A_1,A_2,....,A_{20}$ in that order. Find the number of ways in which three sides of K can be chosen so that every pair among them has at least two sides of K between them. (For example $A_1A_2, A_4A_5, A_{11}A_{12}$ is an admissible triple while $A_1A_2, A_4A_5, A_{19}A_{20}$ is not).
5. Let ABC be a triangle and let $BB_1,CC_1$ be respectively the bisectors of ∠ B, ∠ C with $B_1$ on AC and $C_1$ on AB. Let E, F be the feet of perpendiculars drawn from A onto $BB_1,CC_1$ respectively. Suppose D is the point at which the incircle of ABC touches AB. Prove that AD = EF.
6. Find all pairs (x, y) of real numbers such that $16^{x^2+y}+16^{x+y^2}=1$.