x2+4xy-4y2
=(x^2+4xy+4y^2)-8y^2
=(x+2y)^2-8y^2
=(x+2y+2√2y)(x+2y-2√2y)
x2-2（根号2）x-3
=x^2-2√2x+2-5
=(x-√2)^2-5
=(x-√2+√5 )=(x-√2-√5)