(a) `(a – b)^2 = a^2 + 2ab – b^2`
(b) `(a – b)^2 = a^2 – 2ab + b^2`
(c) `(a – b)^2 = a^2 – b^2`
(d) `(a + b)^2 = a^2 + 2ab – b^2`