The vectors ` vec a` and ` vec b` are not perpendicular and ` vec c` and ` vec d` are two vectors satisfying : ` vec b""xxvec c""= vec b"" xxvec d"",vec a * vec d=0` . Then the vector ` vec d` is equal to : (1) ` vec b-(( vec bdot vec c)/( vec adot vec d)) vec c` (2) ` vec c+(( vec adot vec c)/( vec adot vec b)) vec b` (3) ` vec b+(( vec bdot vec c)/( vec adot vec b)) vec c` (4) ` vec c-(( vec adot vec c)/( vec adot vec b)) vec b`