25(x +y)^-9(x-y)^2
=(5x+5y)^2-(3x-3y)^2
=(5x+5y+3x-3y)(5x+5y-3x+3y)
=(8x+2y)(2x+8y)
=4(4x+y)(x+4y)
平方差公式,不是十字相乘啊