Consider the special case of Identity (IV) with a = b, what do you get? Is it related to Identity (1)?
When a = b ( each = y)
(x + a)(x + b) = x2+ (a + b) x + ab becomes
(x + y)(x + y) = x2 + (y +y)x + (y x y)
= x2 + (2y)x + y2
= x2 + 2yx + y2
Yes it is the same as identity 1.
Consider, the special case of Identity (IV) with a = –c and b = –c. What do you get? Is it related to Identity (II)?