Try to solve these interesting INMO 2007 Questions.

- In a triangle ”ABC” right-angled at ”C”, the median through ”B” bisects the angle between ”BA” and the bisector of ”\(\angle B\)”. Prove that \(\frac{5}{2} < \frac{AB}{BC} < 3 \).
- Let ”n” be a natural number such that \( n = a^2 +b^2 +c^2 \) , for some natural numbers \( a, b, c \). Prove that \( 9n = (p_{1a} + q_{1b} + r_{1c})^2 + (p_{2a} + q_{2b} + r_{2c})^2 + (p_{3a}+ q_{3b} + r_{3c})^2 \),where \(p_{j} ‘s\), \(q_{j} \)’s, \( r_{j}\)’s are all nonzero integers. Further, of 3 does not divide at least one of ”a, b, c,” prove that 9n can be expressed in the form \( x^2 + y^2 + z^2 \), where ”x, y, z” are natural numbers none of which is divisible by 3.
- Let ”m” and ”n” be positive integers such that the equation \( x^2−mx+n = 0 \) has real roots \(\alpha\) and \(\beta \). Prove that \(\alpha \) and \(\beta\) are integers if and only if \([m\alpha]+[m\beta] \) is the square of an integer. (Here [x] denotes the largest integer not exceeding x.)
- Let \(\sigma = (a_{1}, a_{2}, a_{3}, . . . , a_{n}\) be a permutation of (1, 2, 3, . . . , n) . A pair \(a_{i}, a_{j}\) is said to correspond to an inversion of \(\sigma \), if < j but \(a_{i} > a_{j}\) . (Example: In the permutation (2, 4, 5, 3, 1) , there are 6 inversions corresponding to the pairs (2, 1), (4, 3), (4, 1), (5, 3), (5, 1), (3, 1) How many permutations of (1, 2, 3, . . . , n), (n>3) , have exactly two inversions.
- Let ABC be a triangle in which AB = AC. Let D be the midpoint of BC and P be a point on AD. Suppose E is the foot of the perpendicular from P on AC. If \(\frac{AP}{PD}=\frac{BP}{PE}= \lambda\),\(\frac{BD}{AD}= m\) and \(z = m^2(1 + \lambda) \), prove that \(z^2− (\sigma^3− \sigma^2− 2)z + 1 = 0\). Hence show that \(\sigma ≥ 2 \) and \(\lambda = 2\) if and only if ABC is equilateral.
- If x, y, z are positive real numbers, prove that \( (x+y+z)^2(yz+zx+xy)^2≤ 3(y^2+yz+z^2)(z^2+zx+x^2)(x^2+xy+y^2) \).
- Let f :Z mapsto Z be a function satisfying \(f(0) \neq 0\) , \(f(1)=0\) and 1.f(xy) + f(x)f(y) = f(x) + f(y)\), 2. \((f(x-y) – f(0))f(x)f(y) = 0\) for all x , y in Z simultaneously. 1. Find the set of all possible values of the function f. 2. If \(f(10) \neq 0\) and f(2) = 0 , find the set of all integers n such that \( f(n)\neq 0\) .

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