例如：已知关于x、y的多项式（2x^2+mx-1/2 y+3)-(3x-2y+1-nx^2)的值与字母x的取值无关.求多项式（m+2n)-(2m-n)的值.（2x^2+mx-1/2 y+3)-(3x-2y+1-nx^2)=2x^2+mx-1/2 y+3-3x+2y-1+nx^2=（2+n)x^2+(m-3)x+3/2 y+2因为...