（1）已知m+n=3,mn=2/3,求m^3n-m^2n^2+mn^3
问题描述：
（1）已知m+n=3,mn=2/3,求m^3n-m^2n^2+mn^3
(2已知a+b=7,ab=3,求a^3b+ab^3
答
(1)把mn提取出来得：mn(m^2-mn+n^2) .然后(m+n)^2=m^2+n^2+2mn=9.m^2-mn+n^2=7.所以最后为14/3
(2)同样提取ab得ab(aa+bb).最后答案为129