f'(x)=2x+m+ (n/x)=(2x+3m)(x-m)/x (x>0)
极值点x=-3m/2(负数舍去) x=m
x∈(0,m)减 x∈(m,+∞)增
f(x)最小值=f(m)=2m²-3m²*lnm