m^2+2n^2+m-4/3n+17/36=0
(m^2+m+1/4)+2(n^2-2/3n+1/9)=0
(m+1/2)^2+2(n-1/3)^2=0
平方大于等于0,相加等于0,若有一个大于0,则另一个小于0,不成立.
所以两个都等于0
所以m+1/2=0,n-1/3=0
m=-1/2,n=1/3
-mn^2=-(-1/2)*(1/3)^2=(1/2)*(1/9)=1/18