Maths Olympiad Prep

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Geometry Difficulty 5.6 AIME, harder Find the answer

A regular hexagon ABCDEFA B C D E F has side length 1 and center OO. Parabolas P1,P2,,P6P_{1}, P_{2}, \ldots, P_{6} are constructed with common focus OO and directrices AB,BC,CD,DE,EF,FAA B, B C, C D, D E, E F, F A respectively. Let χ\chi be the set of all distinct points on the plane that lie on at least two of the six parabolas. Compute XχOX\sum_{X \in \chi}|O X| (Recall that the focus is the point and the directrix is the line such that the parabola is the locus of points that are equidistant from the focus and the directrix.)

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Recall the focus and the directrix are such that the parabola is the locus of points equidistant from the focus and the directrix. We will consider pairs of parabolas and find their points of intersections (we label counterclockwise): (1): P1P2P_{1} \cap P_{2}, two parabolas with directrices adjacent edges on the hexagon (sharing vertex AA ). The intersection inside the hexagon can be found by using similar triangles: by symmetry this XX must lie on OAO A and must have that its distance from ABA B and FAF A are equal to OX=x|O X|=x, which is to say sin60=32=xOAx=x1xx=233\sin 60^{\circ}=\frac{\sqrt{3}}{2}=\frac{x}{|O A|-x}=\frac{x}{1-x} \Longrightarrow x=2 \sqrt{3}-3 By symmetry also, the second intersection point, outside the hexagon, must lie on ODO D. Furthermore, XX must have that its distance ABA B and FAF A are equal to OX|O X|. Then again by similar triangles sin60=32=xOA+x=x1+xx=23+3\sin 60^{\circ}=\frac{\sqrt{3}}{2}=\frac{x}{|O A|+x}=\frac{x}{1+x} \Longrightarrow x=2 \sqrt{3}+3 (2): P1P3P_{1} \cap P_{3}, two parabolas with directrices edges one apart on the hexagon, say ABA B and CDC D. The intersection inside the hexagon is clearly immediately the circumcenter of triangle BOCB O C (equidistance condition), which gives x=33x=\frac{\sqrt{3}}{3} Again by symmetry the XX outside the hexagon must lie on the lie through OO and the midpoint of EFE F; then one can either observe immediately that x=3x=\sqrt{3} or set up sin30=12=xx+3x=3\sin 30^{\circ}=\frac{1}{2}=\frac{x}{x+\sqrt{3}} \Longrightarrow x=\sqrt{3} where we notice 3\sqrt{3} is the distance from OO to the intersection of ABA B with the line through OO and the midpoint of BCB C. (3): P1P4P_{1} \cap P_{4}, two parabolas with directrices edges opposite on the hexagon, say ABA B and DED E. Clearly the two intersection points are both inside the hexagon and must lie on CFC F, which gives x=32x=\frac{\sqrt{3}}{2} These together give that the sum desired is 6(233)+6(23+3)+6(33)+6(3)+6(32)=3536(2 \sqrt{3}-3)+6(2 \sqrt{3}+3)+6\left(\frac{\sqrt{3}}{3}\right)+6(\sqrt{3})+6\left(\frac{\sqrt{3}}{2}\right)=35 \sqrt{3}

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