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KS3, GCSE, A-Level Computing Resources

Lesson 2. Boolean Algebra


Lesson Objective

  • Be familiar with the use of Boolean identities and De Morgan’s laws to manipulate and simplify Boolean expressions.
  • Write a Boolean expression for a given logic gate circuit, and vice versa.
  • Use the following rules to derive or simplify statements in Boolean algebra:
    1. De Morgan’s Laws
    2. Distribution
    3. Association
    4. Commutation
    5. Absorption
    6. Double negation

Lesson Notes

Boolean Algebra?

Boolean algebra is a mathematical system for working with logical values (true and false) and the operations that combine them. It's a formal method used to manipulate and simplify logical statements, which is very important in computer science and digital circuit design. Rules for simplifying Boolean expressions, like absorption (X ⋀ (X ⋁ Y) = X) and general (X ⋁ ¬X = 1), help us reduce complex expressions to simpler forms that are easier to understand and use.

De Morgan's laws are a key tool for handling negations of combined terms. Specifically, they state that the opposite of "A or B" is the same as "not A and not B," and the opposite of "A and B" is the same as "not A or not B." These laws are crucial for simplifying and changing Boolean expressions, which often results in more efficient ways of implementing them.


Rules

General Rules

  1. X ⋀ 0 = 0
  2. X ⋀ 1 = X
  3. X ⋀ X = X
  4. X ⋀ ¬X = 0
  5. X ⋁ 0 = X
  6. X ⋁ 1 = 1
  7. X ⋁ X = X
  8. X ⋁ ¬X = 1
  9. ¬¬X = X

Commutative Rules

  1. X ⋀ Y = Y ⋀ X
  2. X ⋁ Y = Y ⋁ X

Associative Rules

  1. X ⋀ (Y ⋀ Z) = (X ⋀ Y) ⋀ Z
  2. X ⋁ (Y ⋁ Z) = (X ⋁ Y) ⋁ Z

Distributive Rules

  1. X ⋀ (Y ⋁ Z) = X ⋀ Y ⋁ X ⋀ Z
  2. (X ⋁ Y) ⋀ (W ⋁ Z) = X ⋀ W ⋁ X ⋀ Z ⋁ Y ⋀ W ⋁ Y ⋀ Z

Absorption Rules

  1. X ⋀ (X ⋁ Y) = X
  2. X ⋁ (X ⋀ Y) = X