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From the given logic diagram: Derive Boolean expression and draw the truth table for the derived…

Computer Science20255 marksLong answer
From the given logic diagram:
Figure for this question
(a)[4.0]
Derive Boolean expression and draw the truth table for the derived expression.
(b)[1.0]
If $A = 1$, $B = 0$ and $C = 1$ then find the value of $X$.

Answer

Answer (a)

Official answer key
From the logic diagram: Gate 1 is a NAND gate with inputs A and B, giving output (1) = $(A.B)'$. Gate 2 is an AND gate with inputs B' and C, giving output (2) = $B'.C$. Gate 3 is an OR gate combining (1) and (2), giving output (3) = $(A.B)' + B'.C$. The final gate is a NOT gate acting on (3), giving output $X = [(A.B)' + B'.C]'$. Truth table:
ABC(A.B)'B'.C(A.B)'+B'.CX
0001010
0011110
0101010
0111010
1001010
1011110
1100001
1110001

Answer (b)

Official answer key
$X = [(A.B)'+B'.C]'$. For $A=1, B=0, C=1$: $A.B = 1.0 = 0 \Rightarrow (A.B)'=1$. $B'=1 \Rightarrow B'.C = 1.1 = 1$. $(A.B)'+B'.C = 1+1 = 1$. $X = 1' = 0$. Final answer: X = 0

Final answer: 0

Boolean Algebra

From ISC 2025 Specimen Computer Science Paper 1, question 4(ii).