Each center of the face of the side is a centroid. Say I is the midpoint of of side AC. We can form equilateral triangle FIH. We see that FH = FI = 1/3 of the centroid. Thus each length of the smaller tetrahedron must be 1/3 that of the larger tetrahedron. \( \left (1 \over 3 \right )^3 * 18 = \left (2 \over 3 \right )\)
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