There is a special case example for multiplying expressions with parentheses called the difference of squares.
Consider this multiplication:
(a + b)(a - b)
If we FOIL this we get:
(a + b)(a - b)
equals
a2 - ab + ab - b2
Notice that -ab cancels with +ab. Both of these terms, therefore, drop out, and we are left with:
(a + b)(a - b)
equals
a2 - b2
a2 - b2 is called a difference of squares because it is a subtraction, or difference, of two terms each of which are squared.
So, we can go back and forth each way:
a2 - b2 = (a + b)(a - b)
and
(a + b)(a - b) = a2 - b2