下面给出的或非门组合逻辑电路的输出 f 的布尔表达式是什么? 
(A) (Q+R)’
(B) (P+Q)’
(C) (P+R)
(D) (P+Q+R)’答案：(一)
解释：答案是选项A。
上面的问题包含或非门。让我们看看 NOR 门是做什么的。
如果 A 和 B 是 NOR 门的两个输入，则 NOR 门给出 (A+B)’ 作为输出。
为了便于理解，让我们为盖茨分配数字。
In the 1st column there are 4 NOR Gates,
number them as 1 to 4 ( top to down).
In the 2nd column there are 2 NOR Gates,
number them as 5 and 6 ( top to down).
In the 3rd column there is only 1 NOR Gate,
number it as 7.
1st numbered Gate gives output as : ( P + Q )'
2nd numbered Gate gives output as : ( Q + R )'
3rd numbered Gate gives output as : ( P + R )'
4th numbered Gate gives output as : ( R + Q )'
5th numbered Gate gives output as :
(( P + Q )' + ( Q + R )')'
= ((P + Q)'' . ( Q + R )'') ( De Morgan's law)
= (P + Q ) . ( Q + R ) ( Idempotent Law, A'' = A)
= (PQ + PR + Q + QR )
= (Q(1 + P + R) + PR) = Q + PR ( as, 1 + " any
boolean expression" = 1 )
Similarly 6th numbered Gate gives output as : R + PQ
(as this time R is common here)
Now 7th numbered Gate gives output as :
((Q + PR) + (R + PQ))'
= (Q( 1+P) + R(1+P))'
= (Q+R)' 这个问题的测验