①(x+3y)^2(x-3y)^2
=（(x+3y)(x-3y)）^2
=(x^2-9y^2)^2
=x^4+81y^4-18x^2y^2
②(0.5m+n)(0.5m-n)(0.25m^2-n^2)
=(0.25m^2-n^2)(0.25m^2-n^2)
=0.625m^4-0.5m^2n^2+n^4