(x^2+mx+n)(x^2-4x)展开后含x项为:-4nx
(x^2+mx+n)(x^2-4x)展开后含x^2项为:(n-4m)x
(x^2+mx+n)(x^2-4x)展开后不含x和x^2项
即-4n=0,n-4m=0
n=0
m=0