\(\eqalign{
& \left( {a + b} \right)\left( {a - b} \right) \cr
& = a\left( {a - b} \right) + b\left( {a - b} \right) \cr
& = a.a + a.\left( { - b} \right) + b.a + b.\left( { - b} \right) \cr
& = {a^2} - ab + ab - {b^2} \cr
& = {a^2} + \left( { - ab + ab} \right) - {b^2} \cr
& = {a^2} - {b^2} \cr} \)
