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Edexcel 1CP2 · Topic 2 · 2.3b
Truth Tables &
Boolean Expressions
Constructing Truth Tables · Combined Expressions · Boolean Algebra
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Edexcel GCSE Computer Science 1CP2
Boolean Expressions
Writing Logic as Algebra
A
Boolean expression
uses variables (A, B, C) and operators (AND, OR, NOT) to describe logical conditions. The result is always True (1) or False (0).
Standard notation: A AND B written as
A · B
or just
AB
A OR B written as
A + B
NOT A written as
Ā
(A with bar) or
¬A
Brackets follow standard algebraic precedence; NOT binds tightest
Constructing a Truth Table
Q = (A OR B) AND (NOT A)
A
B
NOT A
A OR B
Q
0
0
1
0
0
0
1
1
1
1
1
0
0
1
0
1
1
0
1
0
Work through intermediate columns first — always show your working in Edexcel exams
3-Input Truth Tables
More Inputs = More Rows
For
n inputs
, a truth table has
2ⁿ rows
: 2 inputs → 4 rows; 3 inputs → 8 rows; 4 inputs → 16 rows.
To fill systematically: the rightmost column alternates 0,1 every row. Next column alternates 0,0,1,1. Leftmost column is 0s then 1s.
Example: Q = A AND B AND C → output is 1 only when A=1, B=1, C=1 (row 8 of 8)
Exam Practice
Have a go at this question
Edexcel-style question
Complete the truth table for: Q = (NOT A) AND B
List all four input combinations and the output.
4 marks
A
B
NOT A
Q
0
0
1
0
0
1
1
1
1
0
0
0
1
1
0
0
[1 per correct row]
Key Takeaways
What to Remember
2ⁿ rows for n inputs — 2 inputs: 4 rows; 3 inputs: 8 rows
Build intermediate columns first to avoid errors on combined expressions
A·B = AND; A+B = OR; Ā or ¬A = NOT
Always show intermediate working — Edexcel can award method marks