2x^2-4xy-5y^2
=2x^2-4xy+2y^2-7y^2
=2(x^2-2xy+y^2)-7y^2
=2(x-y)^2-7y^2
=[√2(x-y)-√7y][√2(x-y)+√7y]
=(√2x-√2y-√7y)(√2x-√2y+√7y)