1997 IMO Problems/Problem 1

Problem

In the plane the points with integer coordinates are the vertices of unit squares. The squares are colored alternatively black and white (as on a chessboard).

For any pair of positive integers $m$ and $n$ , consider a right-angled triangle whose vertices have integer coordinates and whose legs, of lengths $m$ and $n$ , lie along edges of the squares.

Let $S_{1}$ be the total area of the black part of the triangle and $S_{2}$ be the total area of the white part.

Let $f(m,n)=|S_{1}-S_{2}|$

(a) Calculate $f(m,n)$ for all positive integers $m$ and $n$ which are either both even or both odd.

(b) Prove that $f(m,n) \le \frac{1}{2} max\left\{ m,n \right\}$ for all $m$ and $n$ .

(c) Show that there is no constant $C$ such that $f(m,n)<C$ for all $m$ and $n$ .

Solution

For any pair of positive integers $m$ and $n$ , consider a rectangle $ABCD$ whose vertices have integer coordinates and whose legs, of lengths $m$ and $n$ , lie along edges of the squares.

Let $A$ , $B$ , $C$ , and $D$ , be the lower left vertex, lower right vertex, upper right vertex, and upper left vertex of rectangle $ABCD$ respectively.

Let $T_{1}$ be the total area of the black part of the rectangle and $T_{2}$ be the total area of the white part.

Let $g(m,n)=|T_{1}-T_{2}|$

Part (a):

case: $m$ and $n$ which are both even

Since $m$ and $n$ are both even, the total area of the rectangle $ABCD$ is $m \times n$ which is also even

Since every row has an even number of squares there are equally as many white squares than black squares for each row.

Since every column has an even number of squares there are equally as many white squares than black squares for each column.

This means that in the rectangle there are equal number of white squares and black squares.

Therefore $T_{1}=T_{2}=\frac{mn}{2}$ and $g(m,n)=|T_{1}-T_{2}|=0$

Let $M$ be the midpoint of line $AC$ . Them $M$ is at coordinate $(A_{x}+\frac{m}{2},A_{y}+\frac{n}{2})$ Since both $m$ and $n$ are even, then $M$ has integer coordinates.

Starting with vertex $A$ , because the length of $AB$ is even, then the color for the square inside rectangle $ABCD$ closest to $B$ is the opposite color of the square inside rectangle $ABCD$ closest to $A$ , then starting with vertex $B$ , because the length of $BC$ is even, then the color of the square inside rectangle $ABCD$ closest to $C$ is the opposite color of the square inside rectangle $ABCD$ closest to $B$ . this means that the color of the square inside rectangle $ABCD$ closest to $A$ is the same as the color of the square inside rectangle $ABCD$ closest to $C$ . Likewise, the color of the square inside rectangle $ABCD$ closest to $B$ is the same as the color of the square inside rectangle $ABCD$ closest to $D$ .

This color pattern and the fact that the midpoint $M$ has integer coordinates indicates that triangle $ABC$ has the same color pattern as triangle $CDA$ rotated 180 degrees.

Therefore, the white area in triangle $ABC$ is the same as the white area in triangle $CDA$ and the black area in triangle $ABC$ is the same as the black area in triangle $CDA$ .

Thus $S_{1}=\frac{T_{1}}{2}$ and $S_{2}=\frac{T_{2}}{2}$ , which gives $f(m,n)=\frac{g(m,n)}{2}=0$

Therefore $f(m,n)=0$ when both $m$ and $n$ are even.

case: $m$ and $n$ which are both odd

Since $m$ and $n$ are both odd, the total area of the rectangle $ABCD$ is $m \times n$ which is also odd

Since the total area is odd, then $\frac{mn}{2}$ is not an integer but $\frac{mn+1}{2}$ and $\frac{mn-1}{2}$ are.

This means that in the rectangle there are $\frac{mn+1}{2}$ squares of one color and $\frac{mn+1}{2}$ squares of the other color

$g(m,n)=|T_{1}-T_{2}|=\left| \frac{mn+1}{2}-\frac{mn-1}{2} \right|=1$

Let $M$ be the midpoint of line $AC$ . Them $M$ is at coordinate $(A_{x}+\frac{m}{2},A_{y}+\frac{n}{2})$ Since both $m$ and $n$ are odd, then $M$ has non-integer coordinates coordinates but their decimal portions are both $0.5$ . This means that $M$ lies in the middle of the center square.

Starting with vertex $A$ , because the length of $AB$ is odd, then the color for the square inside rectangle $ABCD$ closest to $B$ is the same color of the square inside rectangle $ABCD$ closest to $A$ , then starting with vertex $B$ , because the length of $BC$ is odd, then the color of the square inside rectangle $ABCD$ closest to $C$ is the same color of the square inside rectangle $ABCD$ closest to $B$ . this means that the color of the square inside rectangle $ABCD$ closest to $A$ is the same as the color of the square inside rectangle $ABCD$ closest to $C$ . Likewise, the color of the square inside rectangle $ABCD$ closest to $B$ is the same as the color of the square inside rectangle $ABCD$ closest to $D$ .

This color pattern and the fact that the midpoint $M$ in in the center of the middle square that triangle $ABC$ has the same color pattern as triangle $CDA$ rotated 180 degrees.

Therefore, the white area in triangle $ABC$ is the same as the white area in triangle $CDA$ and the black area in triangle $ABC$ is the same as the black area in triangle $CDA$ .

Thus $f(m,n)=\frac{g(m,n)}{2}=\frac{1}{2}$

Therefore $f(m,n)=\frac{1}{2}$ when both $m$ and $n$ are odd.

To summarize part (a),

$f(m,n)=\begin{cases} 0 & m,n\;are\;both\;even \\ \frac{1}{2} & m,n\;are\;both\;odd\\ \end{cases}$

(a) Calculate $f(m,n)$ for all positive integers $m$ and $n$ which are either both even or both odd.

(b) Prove that $f(m,n) \le \frac{1}{2} max\left\{ m,n \right\}$ for all $m$ and $n$ .

(c) Show that there is no constant $C$ such that $f(m,n)<C$ for all $m$ and $n$ .

Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.

Page

Toolbox

Search

1997 IMO Problems/Problem 1

Problem

Solution