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Permutations and Combinations
Find the number of permutations of n distinct things taken r together in which 3 particular things must occur together.
$\begin{array}{1 1}(A)\;n-3C_{r-3}(r-2)!3!\\(B)\;nC_r\\(C)\;n-1C_{r-1}\\(D)\;n-2C_{r-3}\end{array} $
cbse
math
class11
ch7
permutations-and-combinations
exercise7-3
exemplar
q9
sec-b
medium
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May 15, 2014
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sreemathi.v
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1 Answer
The number of permutations of n distinct things taken r together =$nC_r$
3 things together =$(n-3)C_{r-3}$
$\Rightarrow (n-3)C_{r-3}(r-2)!3!$
Hence (A) is the correct answer.
answered
May 15, 2014
by
sreemathi.v
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