Y by
a) Prove that any section of a regular tetrahedron by a plane passing through the midpoints of its two crossing edges, divides the tetrahedron into two equal parts.
b) Prove that the statement formulated in part (a) is true for any (not necessarily regular) tetrahedron
b) Prove that the statement formulated in part (a) is true for any (not necessarily regular) tetrahedron