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JEE Mains · Maths · STD 11 - 6. permutation and combination

यदि \({ }^{ n } P _{ r }={ }^{ n } P _{ r +1}\) तथा \({ }^{ n } C _{ r }={ }^{ n } C _{ I -1}\) है, तो \(r\) बराबर है

  1. A \(3\)
  2. B \(1\)
  3. C \(4\)
  4. D \(2\)
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Answer & Solution

Correct Answer

(D) \(2\)

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Detailed explanation

\({ }^{n} \mathrm{p}_{r}={ }^{n} p_{r+1} \Rightarrow \frac{n !}{(n-r) !}=\frac{n !}{(n-r-1) !}\) \(\Rightarrow(n-r)=1\) \({ }^{n} \mathrm{C}_{\mathrm{r}}={ }^{n} C_{\mathrm{r}-1}\) \(\Rightarrow \frac{n !}{r !(n-r) !}=\frac{n !}{(r-1) !(n-r+1) !}\)…
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