朋友们来帮我看看这道题怎么做...

m²-4mn+5n²+n+5=（m²-4mn+4n²)+n²+n+5
=(m-2n)²+n²+n+1/4+19/4
=(m-2n)²+(n+1/2)²+19/4
当前两项都为零时,最小值为19/4