ab=3a+2b
ab-3a=2b
a=2b/(b-3)
即
ab=2b²/(b-3)
∵ab＞0
∴b-3＞0
ab=2b²/(b-3)
=（2(b²-9)+18）/(b-3)
=2(b+3)+18/(b-3)
=2(b-3)+12+18/(b-3)
=2(b-3)+18/(b-3)+12
∵2(b-3)+18/(b-3)≥2√2(b-3)*18/(b-3)=6
∴ab≥6+12=18
即ab最小值为18错的。。唉，确实错了有个地方算错了∵2(b-3)+18/(b-3)≥2√2(b-3)*18/(b-3)=2√36=12∴ab≥12+12=24