(a-b)^2 =a^2-2ab+b^2

 (a-b)²  =a²-2ab+b²

^{proof =}

LHS- 

(a-b)²=

(a-b) (a-b)

a(a-b)-b(a-b)

a²-ab-ab+b²

a²-2ab+b²

^proved