①25x²-y²=(5x+y)(5x-y)②（a-b)(x-y)+(a-b)(x+y）=(a-b)[(x-y)+(x+y)]=2x(a-b)③（x+2y)²-8xy=x²+4xy+4y²-8xy=x²-4xy+4y²=(x-2y)²