Consider the special case of Identity (IV) with a = b, what do you get? Is it related to 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)?
Identity IV is given by
( x + a) (x + b) = x2 + (a + b)x + ab
Replacing 'a' by (-c) and 'b' byr (-c), we have
(x - c) (x - c) = x2 + [(-c) + (-c)] x + [(-c) x (-c)]
= x2 + [-12c]x + (c2)
= x2 - 2cx + c2
Which is same as identity II.