Question

solve using identity (a+b+c)(a+b-c)

Answer 1

$(a+b+c)(a+b-c)\\ \\ = \big[(a+b)+c \big] \times \big[(a+b)-c\big]\\ \\ \text{using identity (x+y)(x-y)= x^2-y^2 }\\ \\ =(a+b)^2 - c^2\\ \\ = a^2+b^2+2ab-c^2\\ \\ = \boxed {a^2 +b^2 -c^2 +2ab}$

Answer 2

By identity (a+b)  (a-b)= a²-b²

we get,

(a+b)²-(c)²

a²+b²+2ab - c²