I've got such a problem. In tetrahedron we're considering the three bisectors of angles which are having common vertex. How to proof that if two of this bisectors are perpendicular that implicates that the third one must be also perpendicular to this two bisectors??
The other thing I used in the proof is that the condition for two vectors to be perpendicular is that their scalar product ("dot product") should be zero.
Does that help to make things clearer? If not, you'll have to give a bit more detail about what it is that you don't understand.