✔ 最佳答案
If a.(bxc) = 0 then vector a is perpendicular to vector bxc.
But vector bxc is perpendicular to vectors b and c and
so a,b and c are coplanar.
If a,b,c are coplanar then a=pb+qc where p and q are scalars.
then a.(bxc) = pb(.bxc)+qc.(bxc)
but b.(bxc)=0 and c.(bxc)=0 so
a.(bxc0=0