SATHEE: Permutations And Combinations Question 90

Permutations And Combinations Question 90

Question: If $ ^{n}{C _{r-1}}=36,{{\ }^{n}}C _{r}=84 $ and $ ^{n}{C _{r+1}}=126 $ , then the value of $ r $ is [IIT 1979; Pb. CET 1993, 2003; DCE 1999, 2000; MP PET 2001]

Options:

A) 1

B) 2

C) 3

D) None of these

Show Answer

Answer:

Correct Answer: C

Solution:

Here $ \frac{^{n}{C _{r-1}}}{^{n}C _{r}} $ = $ \frac{36}{84} $ and $ \frac{^{n}C _{r}}{^{n}{C _{r+1}}}=\frac{84}{126} $ . Þ $ 3n-10r=-3 $ and $ 4n-10r=6 $ On solving, we get $ n=9 $ , $ r=3 $ .

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Permutations And Combinations Question 90

Question: If $ ^{n}{C _{r-1}}=36,{{\ }^{n}}C _{r}=84 $ and $ ^{n}{C _{r+1}}=126 $ , then the value of $ r $ is [IIT 1979; Pb. CET 1993, 2003; DCE 1999, 2000; MP PET 2001]

Options:

Answer:

Solution:

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