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Indian National Math Olympiad, INMO 2015 Problems

This post contains problems from Indian National Mathematics Olympiad, INMO 2015. Try them and share your solution in the comments.

INMO 2015, Problem 1

Let $A B C$ be a right-angled triangle with $\angle B=90^{\circ} .$ Let $B D$ be the altitude from $B$ on to $A C .$ Let $P, Q$ and $I$ be the incentres of triangles $A B D, C B D$ and $A B C$ respectively. Show that the circumcentre of of the triangle $P I Q$ lies on the hypotenuse $A C$.

INMO 2015, Problem 2

For any natural number $n>1$, write the infinite decimal expansion of $1 / n$ (for example, we write $1 / 2=0.4 \overline{9}$ as its infinite decimal expansion, not 0.5 ). Determine the length of the non-periodic part of the (infinite) decimal expansion of $1 / n$.

INMO 2015, Problem 3

Find all real functions $f$ from $\mathbb{R} \rightarrow \mathbb{R}$ satisfying the relation
$$f\left(x^{2}+y f(x)\right)=x f(x+y)$$

INMO 2015, Problem 4

There are four basket-ball players $A, B, C, D .$ Initially, the ball is with $A$. The ball is always passed from one person to a different person. In how many ways can the ball come back to $A$ after seven passes? (For example $A \rightarrow C \rightarrow B \rightarrow D \rightarrow A \rightarrow B \rightarrow C \rightarrow A$ and $A \rightarrow D \rightarrow A \rightarrow D \rightarrow C \rightarrow A \rightarrow B \rightarrow A$ are two ways in which the ball can come back to $A$ after seven passes).

INMO 2015, Problem 5

Let $A B C D$ be a convex quadrilateral. Let the diagonals $A C$ and $B D$ intersect in $P$. Let $P E, P F, P G$ and $P H$ be the altitudes from $P$ on to the sides $A B, B C, C D$ and $D A$ respectively. Show that $A B C D$ has an incircle if and only if
$$\frac{1}{P E}+\frac{1}{P G}=\frac{1}{P F}+\frac{1}{P H}$$

INMO 2015, Problem 6

From a set of 11 square integers, show that one can choose 6 numbers $a^{2}, b^{2}, c^{2}, d^{2}, e^{2}, f^{2}$ such that
$$a^{2}+b^{2}+c^{2} \equiv d^{2}+e^{2}+f^{2} \quad(\bmod 12)$$

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