9(x-y)²-25(x-y)²
=[3(x-y)]²-[5(x-y)]²
=[3(x-y)+5(x-y)][3(x-y)-5(x-y)]
=(8x-8y)(-2x+2y)
=-16(x-y)²
对我的回答有不明白的可以追问!
如果还有其它问题,可以向我提问!