Home » Digital Electronics

Axioms and Laws of Boolean Algebra

Boolean Algebra | Axioms: In this tutorial, we are going to learn about the Axioms and Laws of Boolean Algebra in Digital Electronics. By Saurabh Gupta Last updated : May 10, 2023

Boolean Algebra differs from both general mathematical algebra and binary number systems. In Boolean Algebra, A+A =A and A.A = A, because the variable A has only logical value. It doesn't have any numerical significance. In ordinary mathematical algebra, A+A = 2A and A.A = A2, because the variable A has some numerical value here. Also, in Binary Number System 1+1 = 10, and in general mathematical algebra 1+1 = 2 but in Boolean Algebra 1+1 = 1 itself. Unlike ordinary algebra and Binary Number System here is subtraction or division in Boolean Algebra. We only use AOI (AND, OR and NOT/INVERT) logic operations to perform calculations in Boolean Algebra.

Axioms of Boolean Algebra

There are some set of logical expressions which we accept as true and upon which we can build a set of useful theorems. These sets of logical expressions are known as Axioms or postulates of Boolean Algebra. An axiom is nothing more than the definition of three basic logic operations (AND, OR and NOT). All axioms defined in boolean algebra are the results of an operation that is performed by a logical gate.

Axiom 1: 0.0 = 0    Axiom 6: 0+1 = 1
Axiom 2: 0.1 = 0    Axiom 7: 1+0 = 1
Axiom 3: 1.0 = 0    Axiom 8: 1+1 = 1
Axiom 4: 1.1 = 1    Axiom 9: 0 = 1
Axiom 5: 0+0 = 0    Axiom 10: 1 = 0

Laws of Boolean Algebra Based on Axioms

Based on these axioms we can conclude many laws of Boolean Algebra which are listed below,

1. Commutative Laws

A+B = B+A, and 
A.B = B.A

2. Associative Laws

(A+B) + C = A+(B+C)
(A.B). C = A. (B.C)

3. AND Laws

A.0 = 0
A.1 = A
A.A = A
A.A = 0

4. OR Laws

A+0 = A
A+1 = 1
A+A = A
A+A = 0

5. Complementation Laws

If A = 0 then A = 1
If A = 1 then A = 0 
A̿ = A

6. Distributive Laws

A(B+C) = AB + AC
A + BC = (A+B). (A+C)

7. Idempotence Law

A.A = A, If A=1, then A.A = 1.1 =1 = A and if A=0, then A.A = 0.0 = 0 = A 
A+A = A, If A=1, then A+A = 1+1 =1 = A and if A=0, then A+A = 0+0 = 0 = A

8. Absorption Law

A + A.B = A
A.(A+B) = A

9. De-Morgan's Law

A+B =  A. B
A.B = A + B

10. Consensus Theorem

A) AB + A C + BC = AB + A C

Examples/Proofs on Axioms and Laws of Boolean Algebra

Example 1

LHS     = AB + A C + BC
        = AB + A C + BC (A+A)
        = AB + A C + ABC + ABC
        = AB (1+C) + A C (1+C)
        = AB + A C = RHS

    B) (A+B) (A + C) (B+C) = (A+B) (A + C)

Example 2

LHS     = (A+B) (A + C) (B+C)
        = (AA + AC + BA + BC) (B+C)
        = (AC + BA + BC) (B+C)
        = ABC + ACC + BAB + BAC + BCB + BCC
        = ABC + AC + BA + BAC + BC + BC
        = ABC + AC + BA + BAC + BC
        = AC(B+1) + BA (1+C) + BC
        = AC + BA + BC ............. (Equation 1)

RHS     = (A+B) (A + C)
        = AA + AC + BA + BC
        = AC + BA + BC ............. (Equation 2)

Since, Equation 1 = Equation 2, Hence Consensus Theorem is verified.

---