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Question

If a,b,c are three non-coplanar and p,q,r are reciprocal vectors to a,b and c respectively then (la+mb+nc).(lp+mq+nr) is equal to : (where l, m, n are scalars)

A
l2+m2+n2
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B
lm+mn+nl
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C
0
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D
none of these
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Solution

The correct option is A l2+m2+n2
Given a,b,c are three non-coplanar and p,q,r are reciprocal vectors to a,b and c respectively.
ap=1,aq=0,ar=0
bp=0,bq=1,br=0
cp=0,cq=0,cr=1
Now,
(la+mb+nc).(lp+mq+nr)
=l2(ap)+lm(aq)+ln(ar)+lm(bp)+m2(bq)+mn(br)+ln(cp)+mn(cq)+n2(cr)
=l2+m2+n2
(la+mb+nc).(lp+mq+nr)=l2+m2+n2
Hence, option A.

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