X²+4XY-4Y²= X²+4XY+4Y²-8Y²= (X+2Y)²-(2√2Y)²= (X+2Y+2√2Y)(X+2Y-2√2Y)= [X+(2+2√2)Y][X+(2-2√2)Y] 。
x^2+4xy-4y^2=x^2+4xy+4y^2-8y^2=(X+2Y)^2-8y^2=(X+2Y-2√2Y)(X+2Y+2√2Y)
(x+2y)^2
无法分解！
