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# INMO 2015 Problems | Indian National Maths Olympiad

This post contains the six Indian National Maths Olympiad, INMO 2015 problems. Try to solve these problems.

1. Let ABC be a right-angled triangle with $\angle{B}=90^{\circ}$. Let BD is the altitude from B on AC. Let P, Q and Ibe the incenters of triangles ABD, CBD, and ABC respectively. Show that circumcenter of triangle PIQ lies on the hypotenuse AC.

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

3. Find all real functions $f: \mathbb{R} to \mathbb{R}$ such that $f(x^2+yf(x))=xf(x+y)$

4. There are four basketball 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 $\textbf{seven} moves?$ (for example $A\rightarrow C\rightarrow B\rightarrow D\rightarrow A\rightarrow B\rightarrow C\rightarrow A$ , or $A\rightarrow D\rightarrow A\rightarrow D\rightarrow C\rightarrow A\rightarrow B\rightarrow A$).

5. Let ABCD be a convex quadrilateral. Let diagonals AC and BD intersect at P. Let PE, PF, PG, and PH are altitudes from P on the side AB, BC, CD, and DA respectively. Show that ABCD has a incircle if and only if $\frac{1}{PE}+\frac{1}{PG}=\frac{1}{PF}+\frac{1}{PH}$.

6. Show that from a set of 11 square integers one can select six numbers $a^2,b^2,c^2,d^2,e^2,f^2$ such that $a^2+b^2+c^2 \equiv d^2+e^2+f^2$ (mod 12)