1:已知多项式x^2+2axy-xy^2与多项式3xy-axy^2-y^3的和不含xy项
所以2a=-3 a=-3/2
x^2+2axy-xy^2+3xy-axy^2-y^3
=x^2-xy^2+1/2xy^2-y^3
2:(2mx^2-x^2+3x+1)-(5x^2-4y^2+3x)不含x^2项
所以2mx^2-x^2+3x+1-5x^2+4y^2-3x
(2m-1-5)x^2=0
m=3
2m^3-[3m^2+(4m-5)+5]
=2*3^3-[3*3^2+(4*3-5)+5]
=15