Question 976143
Let z = a+b


(a+b+c)(a+b-c)
(z+c)(z-c)
z^2 - c^2 ... difference of squares rule
(a+b)^2 - c^2
a^2 + 2ab + b^2 - c^2 ... perfect square rule


So, (a+b+c)(a+b-c) expands out to a^2 + 2ab + b^2 - c^2

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