If →a,→b,→c are three non-coplanar and →p,→q,→r are reciprocal vectors to →a,→b and →c respectively then (l→a+m→b+n→c).(l→p+m→q+n→r) 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 Al2+m2+n2 Given →a,→b,→c are three non-coplanar and →p,→q,→r are reciprocal vectors to →a,→b and →c respectively. ⇒→a⋅→p=1,→a⋅→q=0,→a⋅→r=0 →b⋅→p=0,→b⋅→q=1,→b⋅→r=0 →c⋅→p=0,→c⋅→q=0,→c⋅→r=1 Now, (l→a+m→b+n→c).(l→p+m→q+n→r) =l2(→a⋅→p)+lm(→a⋅→q)+ln(→a⋅→r)+lm(→b⋅→p)+m2(→b⋅→q)+mn(→b⋅→r)+ln(→c⋅→p)+mn(→c⋅→q)+n2(→c⋅→r) =l2+m2+n2 ∴(l→a+m→b+n→c).(l→p+m→q+n→r)=l2+m2+n2 Hence, option A.