Maths Olympiad Prep

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Geometry Difficulty 5.9 AIME, harder Prove it

44 Prove or disprove the following statement: There exist two tetrahedra T1T_{1} and T2T_{2} satisfying the following conditions:
(1) The volume of T1T_{1} is greater than the volume of T2T_{2}.
(2) Each face area of T1T_{1} does not exceed any face area of T2T_{2}.

Solution

44 There exists a tetrahedron that satisfies the problem. Construct it as follows: T1T_{1} is a regular tetrahedron with edge length 23274\frac{2}{3} \sqrt[4]{27}, and the area of each of its faces is 34(23274)2=1\frac{\sqrt{3}}{4}\left(\frac{2}{3} \sqrt[4]{27}\right)^{2}=1.
Take a regular triangle ABCA B C with an area greater than 3, and draw a perpendicular line through its center OO. Take any point PP on this perpendicular line, then
SPAB=SPBC=SPCA>SOBC>1. S_{\triangle P A B}=S_{\triangle P B C}=S_{\triangle P C A}>S_{\triangle O B C}>1 .

However, when PP is sufficiently close to OO, the volume of this tetrahedron T2T_{2} can be arbitrarily close to zero.

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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.