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Geometry Difficulty 8.6 Shortlist Find the answer

Let OX,OYOX, OY and OZOZ be three rays in the space, and GG a point "[i]between these rays[/i]" (i. e. in the interior of the part of the space bordered by the angles YOZ,ZOXY OZ, ZOX and XOYXOY). Consider a plane passing through GG and meeting the rays OX,OYOX, OY and OZOZ in the points A,B,CA, B, C, respectively. There are infinitely many such planes; construct the one which minimizes the volume of the tetrahedron OABCOABC.

Solution

To solve for the plane that minimizes the volume of the tetrahedron OABC OABC , where the plane meets the rays OX,OY, OX, OY, and OZ OZ at points A,B, A, B, and C C respectively, we need to strategically place these intersection points. To achieve the minimum volume for the tetrahedron OABC OABC , we should make use of the symmetry and optimal conditions for areas within the geometry of the tetrahedron.

### Step-by-step Solving Process:

1. Understand the Geometry:
- Consider the space divided by the three rays OX,OY, OX, OY, and OZ OZ such that G G is the interior point.
- The plane through G G intersects these rays to form the triangle ABC \triangle ABC .

2. Volume of Tetrahedron:
V=13Base AreaHeight V = \frac{1}{3} \cdot \text{Base Area} \cdot \text{Height}
Here, the base can be any of the faces ABC,OAB,OBC, \triangle ABC, \triangle OAB, \triangle OBC, or OCA \triangle OCA , and the height is the perpendicular from the opposite vertex.

3. Optimal Plane Positioning:
- To minimize the volume, the plane should ideally pass through G G symmetrically such that ABC \triangle ABC has minimal area.
- If A,B, A, B, and C C are equidistant projections from G G , the triangle formed on the plane through G G is almost equilateral.

4. Transformation and Analysis:
- *Symmetry*: By symmetry, an equilateral ABC \triangle ABC would minimize deviation, thus minimizing the total volume for fixed G G .
- Consider each projection is inversely proportional to their respective opposite sides.
- OGOA=OGOB=OGOCOA=OB=OC \frac{OG}{OA} = \frac{OG}{OB} = \frac{OG}{OC} \Rightarrow OA = OB = OC

5. Conclusively:
- The plane that minimizes the volume of the tetrahedron OABC OABC is the one where A,B, A, B, and C C are equidistant from each other, forming an equilateral triangle ABC \triangle ABC based at equal heights from O O .

6. Calculate the Minimal Volume:
- For such equilateral ABC \triangle ABC , calculate the area of the triangle by using uniform distribution, and use it directly to find the volume:
- (13Equilateral Area (trig.)h) minimized  \boxed{\left(\frac{1}{3} \cdot \text{Equilateral Area (trig.)} \cdot h\right) \text{ minimized }}

Thus, the minimum volume configuration is achieved when A,B, A, B, and C C are equally spaced around the ray directions, ensuring ABC \triangle ABC has minimum area for the maximum symmetry relative to O O .

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