The following question has been posed in Yahoo! Answers and although I've proved it for a regular tetrahedron, the general case is much more difficult. Any suggestions?

Here's the question:

Let O be any point within a tetrahedron ABCD. Lines joining the vertices A, B, C, D to O when extended meet the opposite faces in P, Q, R and S respectively. Prove that OP/AP + OQ/BQ + OR/CR + OS/DS = 1.

Here's the original Yahoo! Answers post:

Guys please help me in VECTOR CALCULUS PROBLEM"TETRAHEDRON " given below please? - Yahoo! Answers