a+b =12-4ab ≥ 2√ab
4ab+2√ab-12 ≤ 0
-1/2 ≤√ab ≤ 2/3
ab≥ 0
0≤ ab ≤4/9
4ab =12 -(a+b) ≤ (a+b)^2
(a+b)^2 +(a+b) -12 ≥ 0
a+b ≥ 3 或a+b ≤-4
正数a,b,a+b ≥ 3