$\binom{n}{n-r} + \binom{n}{r+1}$,whenever $0 \le r \le n-1$,is equal to

  • A
    $\binom{n}{r-1}$
  • B
    $\binom{n}{r}$
  • C
    $\binom{n}{r+1}$
  • D
    $\binom{n+1}{r+1}$

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Evaluate the sum: $\left( \binom{21}{1} - \binom{10}{1} \right) + \left( \binom{21}{2} - \binom{10}{2} \right) + \left( \binom{21}{3} - \binom{10}{3} \right) + \dots + \left( \binom{21}{10} - \binom{10}{10} \right) = $

Statement $-1$: $\sum_{r=0}^{n} (r+1) \binom{n}{r} = (n+2) 2^{n-1}$
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