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Geometry Difficulty 5.3 AIME, harder Prove it Greece

We consider 111111 mutually distinct points on the interior or on the circle of a unit disk. Prove that we can find at least 19981998 segments with ends from these points and length less than 3\sqrt{3}.

Solution

We divide the circle into three equal sectors of 120120^\circ such that none of the points belong to their border, with the exception of the center of the circle. If the center of the circle is one of the points, then we consider that it belongs only to one of the sectors. This is possible because the number of the points is finite and the possible selections for the distribution of the circle into three equal sectors are infinite.

Figure 1

Let A,BA, B belong to the same sector which is defined by the radii OKOK and OΛO\Lambda. Then, if OAOA, OBOB intersect the circle at AA', BB' and AOB=ω<60A'OB = \omega < 60^\circ, then we get ABAB=2Rsin(ω)<2Rsin60=23/2=3AB \leq A'B' = 2R \sin(\omega) < 2R \sin 60^\circ = 2 \sqrt{3}/2 = \sqrt{3}. Equality is not possible because the points do not belong to the border of the sector. Hence two arbitrary points of the same sector have distance less than 3\sqrt{3}.

Let, in the three sectors, belong xx, yy, zz points, respectively. Then x+y+z=111x + y + z = 111 and the segments with length less than 3\sqrt{3} are at least
(x2)+(y2)+(z2)=x(x1)+y(y1)+z(z1)2=x2+y2+z21112. \binom{x}{2} + \binom{y}{2} + \binom{z}{2} = \frac{x(x-1) + y(y-1) + z(z-1)}{2} = \frac{x^2 + y^2 + z^2 - 111}{2}.
From Cauchy-Schwarz inequality: x2+y2+z2(x+y+z)23=11123x^2 + y^2 + z^2 \geq \frac{(x + y + z)^2}{3} = \frac{111^2}{3} and
(x2)+(y2)+(z2)11123111=1998. \binom{x}{2} + \binom{y}{2} + \binom{z}{2} \geq \frac{111^2}{3} - 111 = 1998.

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