1、9x+1/4+81x^22、-m^2n^2-16+8mn3、3a^2+12ab+12b^24、-6x^2y-3x^3

1、9x+1/4+81x^2
=(9x+1/2)^2
2、-m^2n^2-16+8mn
=-(m^2n^2-8mn+16)
=-(mn-4)^2
3、3a^2+12ab+12b^2
=3(a^2+4ab+4b^2)
=3(a+2b)^2
4、-6x^2y-3x^3-3xy^2
=-3xy(2xy+x^2+y^2)
=-3xy(x+y)^2