3 a+9 (2b) = 3 a+3 (4b) ≥ 2 √ [3 (a+4b)], if and only if a=4b, take the equal sign.
When a=4b, then, a? b= 16b？ ≥2, therefore, b≥ 1/2, and the minimum value of b is 1/2, so a=2 at this time.
Therefore, 3 A+9 (2b) ≥ 2 √ [3 (2+2)] = 18.
Then, the minimum value is 18.
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