2x²+xy-y²
＝x²+xy+x²-y²
＝(x²+xy)+(x²-y²)
＝x(x+y)+(x+y)(x-y)
＝(x+y)[x+(x-y)]
＝(x+y)(2x-y)
即原式＝(2x-y)(x+y)