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Prove that a necessary and sufficient condition for three vectors a, b and c to be coplanar is that there exist scalars l, m, n not all zero simultaneously such that la + mb + nc =0 .

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Necessary Condition: First let a,b,c be three coplanar vectors. Then one of them is expressable as a linear combination of the other two. Let c=xa+yb for some scalars x,y. Then, c= xa+ yb for some scalars x,y la+mb+nc= 0, where l=x, m=y, n=-1Thus, if a,b,c are coplanar vectors, then there exists scalars l, m, n such that la+mb+nc= 0 where l, m, n are all non zero simultaneously.

Sufficient Condition: Let a, b, c be three vectors such that there exists scalars l, m,n not all zero simulataneously satisfying la+ mb+ nc=0. We have tp prove that a, b, c are coplanar vectors.Now, la+ mb+ nc=0nc = -la- mbc =-1na+ -mnbc is a linear combination of a and b .Hence a, b, c are coplanar vectors.

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