m²-2m-8
=m²-2m+1-9
=(m-1)²-9
(m-1)²>=0
(m-1)²-9>=-9
所以最小值=-9
m²+4m+6
=m²+4m+4+2
=(m+2)²+2
(m+2)²>=0
(m+2)²+2>=2
所以最小值=2
7-m²+2m
=-m²+2m-1+8
=-(m-1)²+8
(m-1)²>=0
-(m-1)²