[1] 3ax^3-108ax=3ax(x^2-36)=3ax(x-6)(x+6)
[2]81a^4-1=(9a^2+1)(9a^2-1)=(9a^2+1)(3a+1)(3a-1)
[3](5a-b)^2+20ab=25a^2-10ab+b^2+20b=25a^2+10ab+b^2=(5a+b)^2
[4]16x^3-4x=4x(x^2-1)=4x(x+1)(x-1)
[5]2(x^2)y-8xy+8y=2y(x^2-4x+4)=2y(x-2)^2
[6]3a(x-y)-5b(y-x)=3ax-3ay-5by+5bx=(3a+5b)x-(3a+5b)y
注:^2就是平方 ^3就是立方,依次类推