A=a^2 +5*b^2-4*a*b+2*b+100
=a^2+(2b)^2-4*a*b+b^2+2*b+1+99
=(a-2b)^2+(b+1)^2+99
a-2b=0,b+1=0时使A为最小值
即a=2b,b=-1,得a=-2