令a=b+m
(b+m)^2-b^2=12
2bm+m^2=12
a^2+b^2+ab
=12+2b^2+(b+m)b
=3b^2+mb+12
=3(b+m/6)^2+12-m^2/12
b=-m/6时有最小值,
此时a平方加b平方加a乘b最小值为12-18/12=10.5