9(x+2y)的2次方-25(x-2y)的2次方=?(8x-4y)(-2x+16y) =-8(2x-y)(x-8y)这一步怎么算过去的

9(x+2y)的2次方-25(x-2y)的2次方
=[3(x+2y)+5(x-2y)][3(x+2y)-5(x-2y)]
=(3x+6y+5x-10y)(3x+6y-5x+10y)
=(8x-4y)(16y-2x)
=8(2x-y)(8y-x)
=-8(2x-y)(x-8y)为什么(8x-4y)(16y-2x)=8(2x-y)(8y-x)(8x-4y)(16y-2x)=4(2x-y)*2(8y-x)=8(2x-y)(8y-x)=-8(2x-y)(x-8y)把常数的公因数提取出来