由题意可得,m+n=19,
所以x^3的系数为
[m(m-1)(m-2)+n(n-1)(n-2)]/(3*2*1)=[m^3+n^3-3(m^2+n^2)+2(m+n)]/6
=[(m+n)(m^2-mn+n^2)-3(m^2+n^2)+2(m+n)]/6
=[16(m^2+n^2)-19mn+38]/6
=[16(m+n)^2-32mn-19mn+38]/6
=[16*19^2-51mn+38]/6
>=[5814-51[(m+n)/2]^2]/6
=[5814-51*(19/2)^2]/6
= 201.875