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Boolean Algebra and Minimization

GATE CSE 2021 Set 1 · Q42

MSQ2 marksMedium

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Consider the following Boolean expression. F=(X+Y+Z)(X‾+Y)(Y‾+Z)F = (X + Y + Z)(\overline{X} + Y)(\overline{Y} + Z) Which of the following Boolean expressions is/are equivalent to F‾\overline{F} (complement of FF)?

Select all that applyOne or more correct

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Solution

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Official answer: B) X Y + Z; C) (X + Z)( Y + Z); D) X Y + Y Z + X Y Z

Boolean Algebra and Minimization

Given the Boolean expression in product-of-sums (POS) form:

F=(X+Y+Z)(X‾+Y)(Y‾+Z)F = (X + Y + Z)(\overline{X} + Y)(\overline{Y} + Z)

First, simplify FF: Using the distributive law on (X‾+Y)(Y‾+Z)(\overline{X} + Y)(\overline{Y} + Z):

(X‾+Y)(Y‾+Z)=X‾ Y‾+X‾Z+YZ(\overline{X} + Y)(\overline{Y} + Z) = \overline{X}\,\overline{Y} + \overline{X}Z + YZ

Multiplying by (X+Y+Z)(X + Y + Z):

F=(X+Y+Z)(X‾ Y‾+X‾Z+YZ)F = (X + Y + Z)(\overline{X}\,\overline{Y} + \overline{X}Z + YZ)
F=XYZ+X‾YZ+YZ+X‾ Y‾Z+X‾Z+YZF = XYZ + \overline{X}YZ + YZ + \overline{X}\,\overline{Y}Z + \overline{X}Z + YZ
F=YZ(X+X‾+1)+X‾Z(Y‾+1)=YZ+X‾Z=Z(X‾+Y)F = YZ(X + \overline{X} + 1) + \overline{X}Z(\overline{Y} + 1) = YZ + \overline{X}Z = Z(\overline{X} + Y)

Now, compute the complement F‾\overline{F} using De Morgan's laws:

F‾=Z(X‾+Y)‾=Z‾+X‾+Y‾=Z‾+XY‾\overline{F} = \overline{Z(\overline{X} + Y)} = \overline{Z} + \overline{\overline{X} + Y} = \overline{Z} + X\overline{Y}

Now evaluate each option:

  • Option (A): (X‾+Y‾+Z‾)(X+Y‾)(Y+Z‾)(\overline{X} + \overline{Y} + \overline{Z})(X + \overline{Y})(Y + \overline{Z}) is the dual of FF with complemented literals, which is not equivalent to F‾\overline{F}.

  • Option (B): XY‾+Z‾X\overline{Y} + \overline{Z} is directly identical to F‾\overline{F}.

  • Option (C): (X+Z‾)(Y‾+Z‾)=XY‾+Z‾(X + \overline{Z})(\overline{Y} + \overline{Z}) = X\overline{Y} + \overline{Z}, which matches F‾\overline{F}.

  • Option (D): XY‾+YZ‾+X‾ Y‾ Z‾=XY‾+Z‾(Y+X‾ Y‾)=XY‾+Z‾(Y+X‾)=XY‾+YZ‾+X‾ Z‾X\overline{Y} + Y\overline{Z} + \overline{X}\,\overline{Y}\,\overline{Z} = X\overline{Y} + \overline{Z}(Y + \overline{X}\,\overline{Y}) = X\overline{Y} + \overline{Z}(Y + \overline{X}) = X\overline{Y} + Y\overline{Z} + \overline{X}\,\overline{Z}. By consensus between XY‾X\overline{Y} and X‾ Z‾\overline{X}\,\overline{Z}, this simplifies to XY‾+Z‾(Y+Y‾)=XY‾+Z‾X\overline{Y} + \overline{Z}(Y + \overline{Y}) = X\overline{Y} + \overline{Z}, which also matches F‾\overline{F}.

Thus, options B, C, and D are correct.

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This Boolean Algebra and Minimization previous year question is Q42 of the GATE CSE 2021 Set 1 paper. Find more GATE CSE PYQs on Boolean Algebra and Minimization in Digital Logic, or browse all GATE CSE PYQs.

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Boolean Algebra and Minimization in GATE CSE

Boolean Algebra and Minimization has 36 GATE CSE PYQs across 23 papers from 2011 to 2026. 6 of them are 2-mark MSQs like this one, and it is the only one in GATE CSE 2021 Set 1.

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Consider the following Boolean expression. F = (X + Y + Z)( X + Y)( Y + Z) Which of the following Boolean expressions is/are equivalent to F (complement of F)?

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