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
