已知m+n=3,mn=2/3,求m^2+(n-mn)^2+2mn-(2m^2)n的值
问题描述：
已知m+n=3,mn=2/3,求m^2+(n-mn)^2+2mn-(2m^2)n的值
答
m^2+(n-mn)^2+2mn-2m^2n
=m^2+n^2-2mn^2+m^2n^2+2mn-2m^2n
=(m^2+2mn+n^2)+m^2n^2-(2m^2n+2mn^2)
=(m+n)^2+(mn)^2-2mn(m+n)
=(m+n-mn)^2
=(3-2/3)
=49/9