Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Prove it Brazil

Let CC be a wooden cube. For each pair (X,Y)(X, Y) of vertices of CC, we cut CC through the plane orthogonal to XY\overline{XY} and passing through its midpoint. Into how many pieces is the cube divided?

Solution

Let's call the plane orthogonal to and passing through the midpoint of a segment AB\overline{AB} the "medial plane" of AB\overline{AB}. There exist three different kinds of medial planes: the medial planes of the edges of CC (there are 3 different such planes); the medial planes of the diagonals of the faces of CC (there are 6 of such planes); the medial planes of the diagonals of CC (3 more planes). They divide CC into triangular pyramids, all of them having the center of CC as vertex. Moreover they divide each face of CC into triangles, that serve as bases for the pyramids. It's easy to see that each face is divided in exactly 16 triangles, so that the total number of pieces is 16×6=9616 \times 6 = 96.

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