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The number of ways of arranging 18 boys ...

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

A

`18!-16!.3!`

B

`16!..^(17)P_(3)`

C

`15!.^(16)P_(3)`

D

none of these

Text Solution

Verified by Experts

The correct Answer is:
D
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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