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Questions  

If the direction cosines  l,m,n of two perpendicular lines are connected by the relations λl+3m5n=0  and  10l2+m25n2=0 then λ5 is

a
±3
b
±1
c
±4
d
±5

detailed solution

Correct option is C

The direction cosines of two lines are related by relations al+bm+cn=0  and ul2+vm2+wn2=0 . If these two lines are perpendicular then the condition is ∑a2v+w=0Herea=λ,b=3,c=−5,u=10,v=1,w=−5Hence, ∑a2v+w=0λ2−4+95+2511=0−4λ2+320=0Therefore,  λ=±45⇒λ5=±4

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