Hi
Is it possible to find 5 vectors in the 3-dimensional space that all the angles between them are greater than 90 degrees?
It is nessecary that no three vectors lie in one plane,so they define 10 planes in the space.Also the scalar product of each pair of vectors is negative.I also tried to write a nontrivial linear combination and apply scalar products on it,to obtain sets of equations.This idoes not fill to be so usefull.