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Geometry Difficulty 7.0 National olympiad, round 2 Prove it

8. C2 (GEO) Let D1,,DnD_{1}, \ldots, D_{n} be closed disks in the plane. (A closed disk is a region bounded by a circle, taken jointly with this circle.) Suppose that every point in the plane is contained in at most 2003 disks DiD_{i}. Prove that there exists disk DkD_{k} that intersects at most 7200317 \cdot 2003-1 other disks DiD_{i}.

Solution

8. Let S S be the disk with the smallest radius, say s s , and O O the center of that disk. Divide the plane into 7 regions: one bounded by disk s s and 6 regions T1,,T6 T_{1}, \ldots, T_{6} shown in the figure. Any of the disks different from S S , say Dk D_{k} , has its center in one of the seven regions. If its center is inside S S then Dk D_{k} contains point O O . Hence the number of disks different from S S having their centers in S S is at most 2002. Consider a disk Dk D_{k} that intersects S S and whose center is in the region Ti T_{i} . Let Pi P_{i} be the point such that OPi O P_{i} bisects the region Ti T_{i} and ! OPi=s3 O P_{i} = s \sqrt{3} . We claim that Dk D_{k} contains Pi P_{i} . Divide the region Ti T_{i} by a line li l_{i} through Pi P_{i} perpendicular to OPi O P_{i} into two regions Ui U_{i} and Vi V_{i} , where O O and Ui U_{i} are on the same side of li l_{i} . Let K K be the center of Dk D_{k} . Consider two cases: (i) KUi K \in U_{i} . Since the disk with the center Pi P_{i} and radius s s contains Ui U_{i} , we see that KPis K P_{i} \leq s . Hence Dk D_{k} contains Pi P_{i} . (ii) KVi K \in V_{i} . Denote by L L the intersection point of the segment KO K O with the circle s s . We want to prove that KL>KPi K L > K P_{i} . It is enough to prove that KPiL>KLPi \angle K P_{i} L > \angle K L P_{i} . However, it is obvious that LPiO30 \angle L P_{i} O \leq 30^{\circ} and LOPi30 \angle L O P_{i} \leq 30^{\circ} , hence KLPi60 \angle K L P_{i} \leq 60^{\circ} , while NPiL=90LPiO60 \angle N P_{i} L = 90^{\circ} - \angle L P_{i} O \geq 60^{\circ} . This implies that KPiLNPiL60KLPi \angle K P_{i} L \geq \angle N P_{i} L \geq 60^{\circ} \geq \angle K L P_{i} ( N N is the point on the edge of Ti T_{i} as shown in the figure). Our claim is thus proved. Now we see that the number of disks with centers in Ti T_{i} that intersect S S is less than or equal to 2003, and the total number of disks that intersect S S is not greater than 2002+62003=720031 2002 + 6 \cdot 2003 = 7 \cdot 2003 - 1 .

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.