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Using Boolean equation, the output 'Y' of the network shown below is equal to :

A chain of gates numbered 1 to n: gate 1 is an AND of X0 and X1, gate 2 an OR with X2, gate 3 an AND with X3, and so on, gate n-1 an AND with Xn-1, and gate n an OR with Xn giving Y

  • A.\( X_{0}X_{1}X_{2}\ldots X_{n} + X_{1}X_{2}.......X_{n} \) \( + X_{2}X_{3}.......X_{n} + X_{n} \)
  • B.\( X_{0}X_{1} + X_{2}X_{3} + ....... + X_{n-1}X_{n} \)
  • C.\( X_{0} + X_{1} + X_{2} + ....... + X_{n} \)
  • D.\( X_{0}X_{1}X_{3}.......X_{n-1} + X_{2}X_{3}X_{5} \) \( .......X_{n-1} + X_{n-2}X_{n-1} + X_{n} \)

Answer: D

The output is \( Y = X_{0}X_{1}X_{3}\ldots X_{n-1} + X_{2}X_{3}X_{5}\ldots X_{n-1} \) \( + \ldots + X_{n-2}X_{n-1} + X_{n} \).

The gates alternate AND and OR. Work along the chain:

\[ \begin{aligned} G_{1} &= X_{0}X_{1} \\ G_{2} &= X_{0}X_{1} + X_{2} \\ G_{3} &= X_{0}X_{1}X_{3} + X_{2}X_{3} \end{aligned} \]

Each AND gate multiplies every existing term by its new input, and each OR gate adds its input as a new term. Every term therefore carries all the AND-gate inputs after it (\( X_{3}, X_{5}, \ldots, X_{n-1} \)), and the final OR adds \( X_{n} \) alone.

All-OR, \( X_{0} + X_{1} + \ldots + X_{n} \), would need every gate to be an OR. The sum of pairs ignores the cascading, and the first option multiplies in inputs such as \( X_{2} \) and \( X_{n} \) that enter through OR gates.

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