4a^2+b^2-8a+6b+30
=4a(a-2)+b(b-6)+30
当a=2 ,b=6 时,有最小值.
4a^2+b^2-8a+6b+30
=4a(a-2)+b(b-6)+30
=0+0+30
=30 .b-6应该是b+6哦~~