A) 5
B) 7
C) 6
D) 4
Correct Answer: B
Solution :
[b] \[^{n+1}{{C}_{3}}{{-}^{n}}{{C}_{3}}=21\] \[\because \,\,\,{{\,}^{n}}{{C}_{r}}{{+}^{n}}{{C}_{r-1}}{{=}^{n+1}}{{C}_{r}}\] \[{{\Rightarrow }^{n}}{{C}_{2}}=21\Rightarrow n=7\]You need to login to perform this action.
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