# 1978 IMO Problems

Problems of the 20th IMO 1978 in Romania.

## Day I

### Problem 1

Let $m$ and $n$ be positive integers such that $1 \le m < n$. In their decimal representations, the last three digits of $1978^m$ are equal, respectively, to the last three digits of $1978^n$. Find $m$ and $n$ such that $m + n$ has its least value.

### Problem 2

We consider a fixed point $P$ in the interior of a fixed sphere $.$ We construct three segments $PA, PB,PC$, perpendicular two by two $,$ with the vertexes $A, B, C$ on the sphere $.$ We consider the vertex $Q$ which is opposite to $P$ in the parallelepiped (with right angles) with $PA, PB, PC$ as edges $.$ Find the locus of the point $Q$ when $A, B, C$ take all the positions compatible with our problem.

### Problem 3

Let $0 a sequence with all its terms positive $.$ The $n^{\text{th}}$ positive integer which doesn't belong to the sequence is $f(f(n))+1.$ Find $f(240).$

## Day II

### Problem 4

In a triangle $ABC$ we have $AB = AC.$ A circle which is internally tangent with the circumscribed circle of the triangle is also tangent to the sides $AB, AC$ in the points $P,$ respectively $Q.$ Prove that the midpoint of $PQ$ is the center of the inscribed circle of the triangle $ABC.$

### Problem 5

Let $f$ be an injective function from ${1,2,3,\ldots}$ in itself. Prove that for any $n$ we have: $\sum_{k=1}^{n} f(k)k^{-2} \geq \sum_{k=1}^{n} k^{-1}.$

### Problem 6

An international society has its members from six different countries. The list of members contain $1978$ names, numbered $1, 2, \dots, 1978$. Prove that there is at least one member whose number is the sum of the numbers of two members from his own country, or twice as large as the number of one member from his own country.

 1978 IMO (Problems) • Resources Preceded by1977 IMO 1 • 2 • 3 • 4 • 5 • 6 Followed by1979 IMO All IMO Problems and Solutions