Handling Boolean Expressions

When marking papers, I often come across mistakes on handling boolean expressions, especially those which involve the use of DeMorgan's Theorem. Students often overlook the importance of brackets, say, when changing from (A . B)' to (A' + B'). For example:





Strictly speaking, this step is already wrong!

The problem of omitting the brackets results in this kind of error.











Another example of omitting the brackets, resulting in wrong factorization.

Almost identical mistake as the one above!










This one made a mistake in factorization, even when brackets are used


Careless mistake.





Another common mistake is inconsistency when applying DeMorgan's Theorem. In the following example, the student is able to apply the theorem correctly fro single variable term, eg.

(B' . D)' = B + D'

but when it comes to a product term of 2 variables, it was neglected.








Up to here, it is still correct.

The term (C' . D) should be taken as a whole, i.e. with a bar across the whole product term.