A binary sequence is an array of 0's and 1's. The number of $n$-digit binary sequences which contain even…

A binary sequence is an array of 0's and 1's. The number of $n$-digit binary sequences which contain even number of 0's is
  1. $2^{n-1}$
  2. $2^n-1$
  3. $2^{n-1}-1$
  4. $2^n

Solution

The required number of ways $=$ The even number of 0 's ie, $\{0,2,4,6, \ldots\}$ $\begin{aligned} & =\frac{n !}{n !}+\frac{n !}{2 !(n-2) !}+\frac{n !}{4 !(n-4) !} \\ & ={ }^n C_0+{ }^n C_2+{ }^n C_4+\ldots=2^{n-1} \end{aligned}$

Asked in: AP EAMCET 2009

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