INMO 2008

  1. Let ”ABC” be a triangle, ”I” its in-centre; \(A_1, B_1, C_1 \) be the reflections of I in BC, CA, AB respectively. Suppose the circum-circle of triangle \(A_1 B_1 C_1 \) passes through A. Prove that \(B_1, C_1, I, I_1 \) are concyclic, where \(I_1 \) is the in-centre of triangle \(A_1 B_1 C_1 \).


  2. Find all triples \((p, x, y) \) such that \(p^x = y^4 + 4 \), where \(p \) is a prime and \(x, y \) are natural numbers.
  3. Let ”A” be a set of real numbers such that ”A” has at least four elements. Suppose ”A” has the property that \(a^2 + bc \) is a rational number for all distinct numbers ”a, b, c” in ”A”. Prove that there exists a positive integer ”M” such that \(a\sqrt{M} \) is a rational number for every ”a” in ”A”.
  4. All the points with integer coordinates in the xy-plane are coloured using three colours, red, blue and green, each colour being used at least once. It is known that the point (0, 0) is coloured red and the point (0, 1) is coloured blue. Prove that there exist three points with integer coordinates of distinct colours which form the vertices of a right-angled triangle.
  5. Let ”ABC” be a triangle; \(\Gamma_A , \Gamma_B , \Gamma_C \) be three equal, disjoint circles inside ”ABC” such that \(\Gamma_A \) touches ”AB” and ”AC”; \(\Gamma_B \) touches ”AB” and ”BC”; and “\(\Gamma_C \)” touches ”BC” and ”CA”. Let \(\Gamma \) be a circle touching circles \(\Gamma_A, \Gamma_B, \Gamma_C \) externally. Prove that the line joining the circumcenter ”O” and the in-centre ”I” of triangle ABC passes through the centre of \(\Gamma \).
  6. Let \(P(x) \) be a given polynomial with integer coefficients. Prove that there exist two polynomials \(Q(x) \) and \(R(x) \), again with integer coefficients, such that
    1. \(P(x)Q(x) \) is a polynomial in \(x^2 \); and
    2. \(P(x)R(x) \)is a polynomial in \(x^3 \).

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