Boolean Logic and Expressions
Boolean Logic · 5 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
Writing Boolean expressions from logic diagrams and simplifying using laws appear regularly.
The reverse problem: you are given an expression and specific values for its variables, and have to compute the output.
Method. (1) Substitute each variable with its given
0or1. (2) Apply operators in precedence order: brackets first, then NOT, then AND, then OR. (3) Write down every intermediate value rather than doing the whole thing in your head.
Evaluating an expression and completing its truth table
What comes up: given a logic expression, you either complete its full truth table (every input combination) or work out the single output for one stated set of input values.
Write: substitute each variable with its 0 or 1, then apply the operators in precedence order — brackets first, then NOT, then AND, then OR — writing down each intermediate value as you go. For a full truth table with three inputs, list all eight input rows (count up in binary from 000) and evaluate the expression on each row.
Watch out: in Boolean notation + means OR, not arithmetic addition, so 1 + 1 = 1 (not 2); AB or A·B means AND; and an overbar, ¬ or ' means NOT. Reading + as ordinary addition is the most common slip.