设A=mx^2+2xy-x;B=3x^2-2nxy+3y;A-B=mx^2+2xy-x-(3x^2-2nxy+3y)=(m-3)x^2+2(1+n)xy-x-3y;多项式mx2+2xy-x与3x2-2nxy+3y的差不含二次项;所以,所有二次项的系数都是0；m-3=0;2(1+n)=0;m=3;n=-1;nm=(-1)*3=-3;...