varianta 1
(2x+y-(2x-y))(2x+y+2x-y)=
(2x+y-2x+y)(2x+y+2x-y)=2y*4x=8xy
varianta 2
(a+b)²-(a-b)²=a²+2ab+b²-(a²-2ab+b²)=a²+2ab+b²-a²+2ab-b²=2ab+2ab=4ab
atunci in cazul nostru avem
4ab= 4*2x*y=8xy
varianta 3
4x²+4xy+y²-(4x²-4xy+y²)=4x²+4xy+y²-4x²+4xy-y²=4xy+4xy=8xy
