1) If a,b, c are three non-coplanar vectors p,q,r are reciprocal vectors , then (la+mb+nc) .(lp+mq+nr) is equal to A) l+m+n B) $l^{3}+m^{3}+n^{3}$ C) $l^{2}+m^{2}+n^{2}$ D) none of these Answer: Option CExplanation:p,q and r reciprocal vectors of a,b and c respectively So , p.r=1, p.b=0=p.c q.a=0,q.b=1,q.c-=0 r.a=0, r.b-=0,r.c=1 $\therefore$ (la+mb+nc).(lp+mq+nr) =$l^{2}+m^{2}+n^{2}$
