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put this solution on YOUR website!FOR EASE OF TYPING LET US DENOTE VECTORS V(a)ETC...BY A AND SCALARS P,Q,R BY p,q,r.
WE ARE GIVEN
D=p(AXB)+q(BXC)+r(CXA)............AND (A,B,C)=1/8
DOT BOTH SIDES WITH C
D.C=p(A,B,C)+q(B,C,C)+r(C,A,C)=p/8
p=8(D.C)......SIMLARLY....q=8(D.A).............r=8(D.B)
HENCE...p+q+r=8{(D.C)+(D.A)+(D.B)}=8(D).(A+B+C)......
SO A IS THE ANSWER.