The regular yellow tetrahedral block is
twice the volume of each of his little brothers.
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Start with a regular octahedron (made from 8 octane pieces) and a regular tetrahedron (the all yellow block).
Sliding them together it is easy to see
the tetrahedron and octahedron have the same height.
Splitting the octahedron, it is easy to see the octane
has half the height of the octahedron,
therefore half the height of the yellow tetrahedral block.
Since the octane and regular tetrahedron have the same base (a yellow equilateral triangle) you know the octane is half the volume of the regular tetrahedron block.
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Since the volume of the octane is the same as the kite and double corner,
we now know the the yellow regular tetrahedron
is twice the volume of each of his little brothers.
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