Pourquoi F + F '= 1?

15

J'ai la fonction: f(x,y,z,w)=wx+yz

J'ai trouvé que sa fonction de complément était: f′(x,y,z,w)=w′y′+w′z′+x′y′+x′z′

Je dois montrer que: f+f′=1 mais je ne vois pas comment le faire.

Il semble qu'il n'y ait rien qui s'annule.

Éditer

Comme suggéré, j'ai maintenant utilisé le théorème de DeMorgan et trouvé ceci:

f+f′=wx+yz+(w+y)′+(w+z)′+(x+y)′+(y+z)′

Mais il me semble toujours que rien ne me rapproche de la réalisation de f+f′=1

boolean-algebra Carl
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6
Astuce: Utilisez la loi de
DeMorgan
11
Soit f soit f 'doit être 1
Chu
4
Vous n'avez que 4 entrées. Si rien d'autre, vous pouvez simplement écrire une table de vérité.
Le Photon du
2
Spehro a raison sur l'argent, mais oui, appliquer DeMorgan comme première étape n'aide pas. Donc, pour développer un peu l'indice de Spehro: la solution consiste à faire une algèbre de base, qui inclut DeMorgan comme étape. En utilisant l'algèbre simple + DeMorgan, vous pouvez transformer la fonction f 'en une négation clairement évidente de f. Le griffonnant sur un morceau de papier, il m'a juste fallu 4 étapes pour le faire.
M. Snrub
1
@ Mr.Snrub la première étape de "J'ai trouvé sa fonction complémentaire" devrait être (wx + yz) ′
OrangeDog

Réponses:

4

Since Carl asked nicely. Starting point:

f(x,y,z,w)=wx+yz
and
f′(x,y,z,w)=w′y′+w′z′+x′y′+x′z′

Suivez les étapes suivantes avec f′ :

f′(x,y,z,w)=w′(y′+z′)+x′(y′+z′)
f′(x,y,z,w)=(w′+x′)(y′+z′)
 DeMorgan:
f′(x,y,z,w)=(wx)′(yz)′
 DeMorgan, encore:
f′(x,y,z,w)=(wx+yz)′
 Alors maintenant, le côté droit def′ est juste la simple négation du côté droit def. Ce qui est un peu anti-climatique, puisque maintenant nous nous appuyons uniquement sur le fait que toute expression x+x′=1 , c'est ce que les gens ont toujours dit à propos de f+f′=1 , mais au moins cela fournit un peu Explication de l'algèbre booléenne pour expliquer pourquoi cela est vrai.

M. Snrub
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I don't understand how you got to the second line without passing by your final answer. Your final answer was my first step: it's just the negation of both sides.
C. Lange
The first two lines are the formulas given by OP. They are the starting point, by definition. I fully agree that the stuff later on may have been part of OP's derivation of those first two formulas. But we don't have that information; we just cannot confirm.
Mr. Snrub
Compris - en supposant que et f ′ ont été donnés dans la question comme OP les a écrits. Je croyais que OP avait déjà essayé d'étendre f ' et ne savait pas où aller à partir de là. ff′f′
C.Lange
41

Le fait est que la fonction f() n'a pas vraiment d'importance . Le fait clé est que sa sortie est une seule valeur binaire.

It is a fundamental fact in Boolean algebra that the complement of a binary value is true whenever the value itself is false. This is known as the law of excluded middle. So ORing a value with its complement is always true, and ANDing a value with its complement is always false.

It's nice that you were able to derive the specific function f′(), but that's actually irrelevant to the actual question!

Dave Tweed
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1
This is known as the law of excluded middle.
BallpointBen
@BallpointBen: Thanks! I added it to my answer.
Dave Tweed
13

All previous answers are correct, and very much in depth. But a simpler way to approach this might be to remember that in boolean algebra, all values must be either 0 or 1.

So... either F is 1, then F' is 0, or the other way around: F is 0 and F' is 1. If you then apply the boolean OR-function: F + F', you will always have one of both terms 1, so the result will always be 1.

Opifex
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11

My answer is similar to the one of Dave Tweed, meaning that I put it on a more formal level. I obviously answered later, but I decided to nevertheless post it since someone may find this approach interesting.

The relation you are trying to prove is independent from the structure of the function f since it is, as a matter of fact, a tautology. To explain what I mean, I propose a demonstration for a general, correctly formed, Boolean expression P in an arbitrary number of Boolean variables, say n∈N, y1,…,yn, where yi∈{0,1} for all i=1,…,n.
We have that P(y1,…,yn)∈{0,1} and consider the following two sets of Boolean values for the n-dimensional Boolean vector (y1,…,yn)

Y={(y1,…,yn)∈{0,1}n|P(y1,…,yn)=1}Y¯={(y1,…,yn)∈{0,1}n|P(y1,…,yn)=0}
These set are a partition of the full set of values the input Boolean vector can assume, i.e. Y∪Y¯={0,1}n and Y∩Y¯=∅ (the empty set), thus
P(y1,…,yn)={0if (y1,…,yn)∈Y¯1if (y1,…,yn)∈Y⇕P′(y1,…,yn)={1if (y1,…,yn)∈Y¯0if (y1,…,yn)∈Y
 therefore we always have
P+P′=1∀(y1,…,yn)∈{0,1}n

Daniele Tampieri
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11

All good answers that provide the necessary justification in one way or the other. Since it is a tautology, it's hard to create a proof that doesn't just result in "it is what it is!". Perhaps this method help tackle it from yet another, broader angle:

Expand both statements to include their redundant cases, and the remove the repeated cases:

𝑓=𝑤𝑥+𝑦𝑧  =wx⋅(y′z′+y′z+yz′+yz) + yz⋅(x′w′+x′w+xw′+xw)  =wxy′z′+wxy′z+wxyz′+wxyz + yzx′w′+yzx′w+yzxw′+yzxw  =wxy′z′+wxy′z+wxyz′+wxyz + yzx′w′+yzx′w+yzxw′

and

𝑓′=𝑤′𝑦′+𝑤′𝑧′+𝑥′𝑦′+𝑥′𝑧′   =w′y′⋅(x′z′+x′z+xz′+xz) + 𝑤′𝑧′⋅(x′y′+x′y+xy′+xy) +         x′y′⋅(w′z′+w′z+wz′+wz) + x′𝑧′⋅(w′y′+w′y+wy′+wy)   =w′y′x′z′+w′y′x′z+w′y′xz′+w′y′xz + 𝑤′𝑧′x′y′+𝑤′𝑧′x′y+𝑤′𝑧′xy′+𝑤′𝑧′xy +         x′y′w′z′+x′y′w′z+x′y′wz′+x′y′wz + x′𝑧′w′y′+x′𝑧′w′y+x′𝑧′wy′+x′𝑧′wy   =w′y′x′z′+w′y′x′z+w′y′xz′+w′y′xz + 𝑤′𝑧′x′y+𝑤′𝑧′xy +         x′y′wz′+x′y′wz + x′𝑧′wy

I've kept the terms in consistent order to make the derivation more obvious, but they could be written alphabetically to be clearer. In any case, the point is that f ORs seven 4-bit cases, and f′ ORs nine, distinct 4-bit cases. Together they OR all sixteen 4-bit cases, so reduce to 1.

Heath Raftery
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4
+1 this is the only answer that is answering the true intention of the OPs question, which is to do some Boolean algebra rather than making theoretical arguments. But per my comment on the OP, note that a more elegant solution does exist; this problem can be solved without needing to add in the redundant cases.
Mr. Snrub
I would very much like to see that as well. That is, if you have the time and the generosity to do it.t
Carl
8

F + F' = 1 means that you have to show that no matter the state of the 4 inputs, OR'ing the result of those 2 always result in 1,

A few minutes in excel shows it is indeed the case. You can use "NOT()" to invert between 0 and 1 in excel.

F = W * X + Y * Z

F' = W' * Y' + W' * Z' + X' * Y' + X' * Z'

As to why this is the case, If you want F to be false, e.g. setting W and Y low, you just made F' true. If you make X and Z low, you also made F" true, same for swapping there pairs.

enter image description here

Reroute
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2
"F + F' = 1 means that you have to show that no matter the state of the 4 inputs, OR'ing the result of those 2 always result in 1" . No, it doesn't. It merely means that you have to show that regardless of the output (which can only have two possibilities) and the corresponding output of its complement, the relation holds. The inputs are irrelevant, as is the function. The only truth table needed is the one showing the relationship between the output of the function and the output of anything qualifying as its complement.
Chris Stratton
@ChrisStratton, that depends if the question is to show that the OR of a function and its complement is always 1 (which is trivial by definition of the complement) or to show that the proposed function F' is actually the complement of F. From OP's wording, I think they had a 2 part problem. Part A: find the complement function. Part B: show that it actually is the complement.
The Photon
0

By simple definition of + (OR) and ′ (NOT)

 A | B | A + B
---------------
 0 | 0 |   0
 1 | 0 |   1
 0 | 1 |   1
 1 | 1 |   1

 A | A′| A + A′
----------------
 0 | 1 |   1
 1 | 0 |   1

∴∀f.f+f′=1

OrangeDog
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