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.

Cheers

Cabouli