IMO 1968

Official IMO 1968 problems  ·  6/6 solved, 3 verified.

Problem 1   solved · 4m22s · Solution →

Prove that there is one and only one triangle whose side lengths are consecutive integers, and one of whose angles is twice as large as another.

Problem 2   ✓ verified · 4m57s · Solution →

Find all natural numbers $x$ such that the product of their digits (in decimal notation) is equal to $x^2 - 10x - 22$.

Problem 3   solved · 9m13s · Solution →

Consider the system of equations $$ ax_1^2 + bx_1 + c = x_2 $$ $$ ax_2^2 + bx_2 + c = x_3 $$ $$ \cdots $$ $$ ax_{n-1}^2 + bx_{n-1} + c = x_n $$ $$ ax_n^2 + bx_n + c = x_1 $$ with unknowns $x_1, x_2, \cdots, x_n$ where $a, b, c$ are real and $a \neq 0$. Let $\Delta = (b - 1)^2 - 4ac$. Prove that for this system

(a) if $\Delta < 0$, there is no solution,

(b) if $\Delta = 0$, there is exactly one solution,

(c) if $\Delta > 0$, there is more than one solution.

Problem 4   solved · 18m05s · Solution →

Prove that in every tetrahedron there is a vertex such that the three edges meeting there have lengths which are the sides of a triangle.

Problem 5   ✓ verified · 4m05s · Solution →

Let $f$ be a real-valued function defined for all real numbers $x$ such that, for some positive constant $a$, the equation $$ f(x + a) = \frac{1}{2} + \sqrt{f(x) - (f(x))^2} $$ holds for all $x$.

(a) Prove that the function $f$ is periodic (i.e., there exists a positive number $b$ such that $f(x + b) = f(x)$ for all $x$).

(b) For $a = 1$, give an example of a non-constant function with the required properties.

Problem 6   ✓ verified · 8m57s · Solution →

For every natural number $n$, evaluate the sum $$ \sum_{k = 0}^\infty\bigg[\frac{n + 2^k}{2^{k + 1}}\bigg] = \Big[\frac{n + 1}{2}\Big] + \Big[\frac{n + 2}{4}\Big] + \cdots + \bigg[\frac{n + 2^k}{2^{k + 1}}\bigg] + \cdots $$ (The symbol $[x]$ denotes the greatest integer not exceeding $x$.)