Maths Olympiad Prep

Library / /409 of 740

Geometry Difficulty 5.0 AIME, harder Prove it United States

Problem:
Let ABCDEFABCDEF be a regular hexagon with center OO and side length 11. Point XX is placed in the interior of the hexagon such that BXC=AXE=90\angle BXC = \angle AXE = 90^\circ. Compute all possible values of OXOX.

Solutions — 2

Solution 1

Solution:
Figure 1
Point XX is the intersection of circles with diameter AEAE and BCBC. Thus, there are two possible intersection points. Since ACBEAC \perp BE, the first point, X1X_1, is the intersection of ACAC and BEBE, from which we can see OX1=12OX_1 = \frac{1}{2} as our first answer. Let X2X_2 be the other intersection point.

Let MM be the midpoint of BCBC and NN be the midpoint of AEAE. Then MX2=NO=12MX_2 = NO = \frac{1}{2} and MO=NX2=32MO = NX_2 = \frac{\sqrt{3}}{2}, so OX2MNOX_2MN is an isosceles trapezoid. By law of cosine on OMN\triangle OMN, we have
MN=OM2+ON22OMONcos150=(32)2+(12)2+2321232=72 \begin{aligned} MN & = \sqrt{OM^2 + ON^2 - 2 \cdot OM \cdot ON \cos 150^\circ} \\ & = \sqrt{\left(\frac{\sqrt{3}}{2}\right)^2 + \left(\frac{1}{2}\right)^2 + 2 \cdot \frac{\sqrt{3}}{2} \cdot \frac{1}{2} \cdot \frac{\sqrt{3}}{2}} = \frac{\sqrt{7}}{2} \end{aligned}
Moreover, by Ptolemy's theorem,
OX2MN=MO2NO2=12 OX_2 \cdot MN = MO^2 - NO^2 = \frac{1}{2}
Combining the previous two equations gives OX2=77OX_2 = \frac{\sqrt{7}}{7}.

Solution 2

Solution:
Recall the first paragraph of the previous solution that X1=ACBEX_1 = AC \cap BE is the first point. Thus, the second point X2X_2 is the Miquel point of cyclic quadrilateral ACBEACBE.

By a well-known property of Miquel point, if Y=ABCEY = AB \cap CE, then YY and X2X_2 are inverses with respect to the circumcircle of ABCDEFABCDEF. Thus, OX2OY=1OX_2 \cdot OY = 1.

One can compute OYOY as follows: from triangle BCYBCY, we get that BY=BC/sin30=2BY = BC / \sin 30^\circ = 2. Thus, by power of point,
OY212=YBYA=23=6OY=7 OY^2 - 1^2 = YB \cdot YA = 2 \cdot 3 = 6 \Longrightarrow OY = \sqrt{7}
implying OX2=77OX_2 = \frac{\sqrt{7}}{7}.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.