Permutations And Combinations Question 107
Question: If $ ^{n}C _{r}=84,{{\ }^{n}}{C _{r-1}}=36 $ and $ ^{n}{C _{r+1}}=126 $ , then $ n $ equals [RPET 1997; MP PET 2001]
Options:
A) 8
B) 9
C) 10
D) 5
Correct Answer: B $ \therefore $ $ \frac{7}{3}r-1=n-r=\frac{3}{2}(r+1) $ or $ 14r-6=9r+9 $ or $ r=3 $ . So, $ n=9 $ .
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Answer:
Solution:
BETA
