मराठी

Combination

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Estimated time: 7 minutes
Maharashtra State Board: Class 12

Formula: Combination

The number of combinations of n different things taken r at a time is

C(n, r) or \[^nC_r\] or \[\begin{pmatrix} n \\ r \end{pmatrix}\] i.e. \[^nC_r=\frac{n!}{r!(n-r)!},0\leq r\leq n\]

Maharashtra State Board: Class 12

Key Points: Combination

  • \[^nC_{n-r}=^nC_r\mathrm{~for~}0\leq r\leq n\]
  • \[{}^{n}C_{0}={}^{n}C_{n}=1\]
  • \[{}^{n}C_{0}={}^{n}C_{n}=1\]
  • If \[^nC_r={}^nC_s\], then either s = r or s = n - r.
  • \[^nC_r=\frac{^nP_r}{r!}\]
  • \[^nC_r+{}^nC_{r-1}={}^{n+1}C_r\]
  • \[^nC_0+{}^nC_1+...+{}^nC_n=2^n\]
  • \[^nC_0+^nC_2+^nC_4+...\] \[={}^{n}C_{1}+{}^{n}C_{3}+{}^{n}C_{5}+....=2^{(n-1)}\]
  • \[^nC_r=\left(\frac{n}{r}\right)^{(n-1)}C_{(r-1)}=\left(\frac{n}{r}\right)\left(\frac{n-1}{r-1}\right)^{(n-2)}C_{(r-2)}=...\]
  • \[^nC_r\] has maximum value, if
    a. \[\mathrm{r=\frac{n}{2}}\], when n is even.
    b. \[\mathbf{r}=\frac{\mathbf{n}-1}{2}\] or \[\frac{\mathrm{n}+1}{2}\], when n is odd.

Shaalaa.com | Theorem: `"^nC_r` + `"^nC_r-1`= `"^(n+1)C_r`

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Theorem: `"^nC_r` + `"^nC_r-1`= `"^(n+1)C_r` [00:06:31]
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