1 9（a-b)^2-16(a+b)^2
=-7a^2-50ab-7b^2
=-(7a+b)(a+7b)
2 3x^3-12x^2y+12xy^2
=3x(x^2-4xy+4y^2)
=3x(x-2y)^2
3 25(x-y)^2+10(y-x)+1
=25(x-y)^2-10(x-y)+1
=[5(x-y)-1]^2
=(5x-5y-1)^2