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Which one of the following is NOT a valid identity ?

  • A.(x⊕y)⊕z = x⊕(y⊕z)
  • B.(x+y)⊕z = x⊕(y+z)
  • C.x⊕y = x + y if xy=0
  • D.x⊕y = (xy+x'y')'

Answer: B

The identity that is not valid is (x+y)⊕z = x⊕(y+z).

Take x = 1, y = 1, z = 0. The left side is (1 + 1)⊕0 = 1⊕0 = 1, while the right side is 1⊕(1 + 0) = 1⊕1 = 0. The two sides differ, so OR and XOR cannot be regrouped this way.

The others hold: XOR is associative; when xy = 0 the two inputs are never both 1, so XOR equals OR; and x⊕y is the complement of XNOR, (xy + x'y')'.

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