5a^2+3ab ; 2x^2-4xy(1)错,第二个减号改为加号(2)错,漏加了m(1)(-3/5)x^3+(4/5)x^2-x(2)-6a^4b^2+4a^2b^3-2a^2b^4-2(1)-12a^3+9a^2(2)(1/4)x^2y^3-x^2y^2(3)-8a^4b^3-(64/3)a^3b^3+(4/3)a^2b^4 (次方看不清楚,此处都作为2来算的)(4)=-a^3b-2a^2b^2-5a^3b+5a^2b^2=-6a^3b+3a^2b^2=3m(2m-3n)+(3m-n)(2m) =12m^2-11mn