Maths Olympiad Prep

Library / /114 of 520

Geometry Difficulty 6.5 National olympiad Find the answer

Given three tetrahedrons AiBiCiDiA_iB_i C_i D_i (i=1,2,3i=1,2,3), planes αi,βi,γi\alpha _i,\beta _i,\gamma _i (i=1,2,3i=1,2,3) are drawn through Bi,Ci,DiB_i ,C_i ,D_i respectively, and they are perpendicular to edges AiBi,AiCi,AiDiA_i B_i, A_i C_i, A_i D_i (i=1,2,3i=1,2,3) respectively. Suppose that all nine planes αi,βi,γi\alpha _i,\beta _i,\gamma _i (i=1,2,3i=1,2,3) meet at a point EE, and points A1,A2,A3A_1,A_2,A_3 lie on line ll. Determine the intersection (shape and position) of the circumscribed spheres of the three tetrahedrons.

A number or a short expression. Spacing and $ signs are ignored.

Solution

1. Understanding the Problem:
We are given three tetrahedrons AiBiCiDi A_iB_iC_iD_i (for i=1,2,3 i=1,2,3 ). For each tetrahedron, planes αi,βi,γi \alpha_i, \beta_i, \gamma_i are drawn through Bi,Ci,Di B_i, C_i, D_i respectively, and these planes are perpendicular to the edges AiBi,AiCi,AiDi A_iB_i, A_iC_i, A_iD_i respectively. All nine planes intersect at a common point E E . Additionally, points A1,A2,A3 A_1, A_2, A_3 lie on a line l l .

2. Analyzing the Perpendicularity:
Since αi \alpha_i is perpendicular to AiBi A_iB_i , βi \beta_i is perpendicular to AiCi A_iC_i , and γi \gamma_i is perpendicular to AiDi A_iD_i , the point E E must be such that:
EBiBiAi,ECiCiAi,EDiDiAi EB_i \perp B_iA_i, \quad EC_i \perp C_iA_i, \quad ED_i \perp D_iA_i
This implies that E E is the orthocenter of the tetrahedron AiBiCiDi A_iB_iC_iD_i .

3. Circumscribed Sphere:
The circumscribed sphere of a tetrahedron is the sphere that passes through all four vertices of the tetrahedron. Given that E E is the orthocenter, the circumscribed sphere of AiBiCiDi A_iB_iC_iD_i has EAi EA_i as its diameter. This is because the orthocenter and the circumcenter of a tetrahedron are related in such a way that the orthocenter lies on the diameter of the circumscribed sphere.

4. Intersection of Spheres:
Since A1,A2,A3 A_1, A_2, A_3 lie on the line l l , and each sphere has EAi EA_i as its diameter, the spheres intersect along a common circle. Let F F be the foot of the perpendicular from E E to the line l l . The diameter of the circle of intersection is EF EF , and this circle lies in the plane perpendicular to l l .

5. Conclusion:
The intersection of the circumscribed spheres of the three tetrahedrons is a circle with diameter EF EF in the plane perpendicular to the line l l .

The final answer is the circle with diameter EF \boxed{ EF } in the plane perpendicular to the line l l .

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