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NDA 2020
Exam
NDA
Year
2020
Subject
Mathematics
Chapter
Permutation and Combination

NDA 2020 · Mathematics · Permutation and Combination

If C(20, n+2)=C(20, n-2) , then what is n equal to?

If C(20,n+2)=C(20,n−2)C(20, n+2)=C(20, n-2), then what is nn equal to?

Options

  1. A18
  2. B25
  3. C10
  4. D12

Correct answer

C. 10

Step-by-step solution

(20(n+2))=(20(n−2))⇒20!(n+2)!(20−n−2)!=20!(n−2)!(20−n+2)!\binom{20}{(n+2)}=\binom{20}{(n-2)}\Rightarrow \frac{20!}{(n+2)!(20-n-2)!}=\frac{20!}{(n-2)!(20-n+2)!}⇒(22−n)!(18−n)!=(n+2)!(n−2)!⇒(22−n)(21−n)(20−n)(19−n)=(n+2)(n+1)⋅n⋅(n−1)\begin{aligned} & \Rightarrow \frac{(22-n)!}{(18-n)!}=\frac{(n+2)!}{(n-2)!} \\ & \Rightarrow(22-n)(21-n)(20-n)(19-n) \\ & \quad=(n+2)(n+1) \cdot n \cdot(n-1)\end{aligned}For $n=10\begin{aligned} & (22-10)(21-10)(20-10)(19-10) \\ & =(10+2)(10+1) \cdot 10 \cdot(10-1) \\ & \Rightarrow 12 \cdot 11 \cdot 10 \cdot 9=12 \cdot 11 \cdot 10 \cdot 9 \end{aligned}$$Hence,Hence,n=10$.
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