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Question

If r be a vector perpendicular to a+b+c, where [a b c]=z and r=(b×c)+m(c×a)+n(a×b), then find l+m+n.

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Solution

Given that r a+b+c
So r.(a+b+c+)=0
r.a+r.b+r.c=0 ......... 1

and z=[a b c]
Given r=l(b×c)+(c×a)+n (a×b)
Now r.a=l a.(b×c)+m a.(a×a)+n a.(a×b)
r.a=lz+m×0+n×0
y.a=lz ........ 2
r.b=l b(b×c)+m b.(c×a)+nb.(a×b)
=l(0)+mz+n×0
r.b=mz ....... 3
r.c=l c.(c×a)+nc.(a×b)
r.c=l(0)+m(0)+n(z)
r.c=nz ........... 4
Now Eq.2+Eq.3+Eq.4
r.a+r.b+r.c=lz+mz+nz
by Eq.1
r.a+r.b+r.c=0
0=z(l+m+n)
l+m+n=0











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