(х+2у)^2-(x-2y)^2=8xy
x^2+4xy+y^2-(x^2-4xy+y^2)=8xy
x^2+4xy+y^2-x^2+4xy-y^2=8xy
4xy+4xy=8xy
8xy=8xy
ДОКАЗАНО
(х+2у)^2-(x-2y)^2=8xy
x^2+4xy+y^2-(x^2-4xy+y^2)=8xy
x^2+4xy+y^2-x^2+4xy-y^2=8xy
4xy+4xy=8xy
8xy=8xy
ДОКАЗАНО