## Precalculus (6th Edition)

$(x + y)^2$ - $(x - y)^2$ Now, $a^2$ - $b^2$ = (a+b)(a-b) and a = x+y , b = x-y Which implies, = ((x+y) + (x-y))((x+y) - (x-y)) = (2x)(2y) = 4xy which is the answer