The formula of the second method is A-B=AB* (the mobile phone code can't display the above bar, so * is used instead).

C-(A∪B)= C ∪( A∪B)* = CA * B *

A B-CA * B * = AB∩(CA * B *)* = AB ∩( C *∪A∪B)= ABC *∪AB∪AB = AB