SATHEE: Permutations And Combinations Question 105

Permutations And Combinations Question 105

Question: If $ ^{2n}C_2{{:}^{n}}C_2=9:2 $ and $ ^{n}C _{r}=10 $ , then $ r= $

Options:

A) 1

B) 2

C) 4

D) 5

Show Answer

Answer:

Correct Answer: B

Solution:

$ ( \frac{(2n)\ !}{2\ !\ (2n-2)\ !} )2=( \frac{n\ !}{2\ !(n-2)\ !} )9 $

$ \Rightarrow (2n)(2n-1)2=9n(n-1)\Rightarrow n=5 $ Now $ ^{5}C _{r}=10\Rightarrow r=2 $ .

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Permutations And Combinations Question 105

Question: If $ ^{2n}C_2{{:}^{n}}C_2=9:2 $ and $ ^{n}C _{r}=10 $ , then $ r= $

Options:

Answer:

Solution:

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