Here is the question
"Two pairs of opposite edges of a tetrahedron are perpendicular. Prove that the third pair is also perpendicular."
I let the 3 sides be a, b and c
assume a is perpendicular to b
assume b and c are perpendicular
then a.b=0 and b.c=o
need to show a.c=o
but I am not sure how to proceed.