原式=X平方-XY-XY- Y平方=X平方-2XY- Y平方=X平方-2XY+ Y平方-2y平方=（X-Y）平方-2y平方=（x-y+根下2y）（x-y-根下2y）
原式=x^2-xy-yx-y^2=x^2-2xy-y^2
x(x-y)-y(x+y)＝x²-xy-xy-y²=x²-2xy-y²=(x²-2xy+y²)-y²-y²=(x-y)²-2y²=(x-y+√2y)(x-y-√2y)
