1/a+2/b =3
(2a+b)/ab=3
2a+b=3ab
3ab=2a+b≥2根号(2ab)
ab≥8/9
（a+1)(b+2)
=ab+2a+b+2
=4ab+2
≥32/9+2=50/9
所以
（a+1)(b+2)的最小值是50/9