Rectangles

**Defunkt** · August 30th 2009, 01:31 PM

Let $R = [a,b]\times[c,d]$ be a rectangle $( d(ab) = d(cd); d(ac) = d(bd) )$ and let $R_i = (a_i,b_i)\times(c_i,d_i)$ , ( $1\leq i \leq n$ ) be rectangles inside R, such that every two rectangles in R are disjoint, maybe other than their sides, and such that each one of them has at least one integer side.

Also,

$\bigcup_{1\leq i \leq n}R_i = R$

Prove that R also has at least one integer side.

**Bruno J.** · August 30th 2009, 11:45 PM

We'll consider the whole thing as a possibly quite irregular grid on which we will move. A "point" will mean the vertex of some rectangle, and "moving up once" will mean to move up from a point to the next point directly above, along a side of integer length.

First notice that if a rectangle has one side of integer length then the opposite side is also of integer length. Therefore, given two concurrent sides of a rectangle, one will be of integer length.

Start from the lower left corner. From there you can certainly move up or move right; one of those sides has to be of integer length. From the point you reach then, the same is possible : you can move either up or right. Keep doing that until you reach either the right side or the top side of $R$ . If you reach the right side, then $R$ has integer width (add up all the right-left travel). Similarily if you reach the top side, then $R$ has integer height. Since one of the two must happen, we are done.

**Defunkt** · August 31st 2009, 02:03 AM

Very nice! I haven't quite thought about it this way. There are numerous other ways of solving this, so anyone is welcome to have a shot!

Another nice solution is by using the fact that:

$\sum^{n}_{i=1} \int_{R_i} sin(2\pi x)sin(2\pi y)dxdy = \int_{R}sin(2\pi x)sin(2\pi y) dxdy$

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