1.

The number of ways of arranging 18 boys so that 3 particular boys are separated is :

Answer»

<P>`18!-16!.3!`
`16!..^(17)P_(3)`
`15!.^(16)P_(3)`
NONE of these

Solution :We have `|vec(a)+vec(B)+vec(c)+vec(d)|^(2)ge 0 implies4+2Sigmavec(a).vec(b)ge0implies-2Sigmavec(a).vec(b)le4`
`Sigma|vec(a)-vec(b)|^(2)=3[Sigma|vec(a)|^(2)]-2.Sigmavec(a).vec(b)le3(4)+4=16`


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