The expression [(a + b)^2]/4 - [a^2b^2]/9 can be factored as a difference of squares. It can be rewritten as (1/36)((3(a+b))^2 - (2ab)^2). By applying the difference of squares formula, further simplification can be achieved. This approach provides a clear pathway to factor the given fractional expression effectively. The discussion highlights the importance of recognizing patterns in algebraic expressions for easier manipulation.