∵3是3^a与3^b的等比中项，∴3²=3^a•3^b=3^a+b，∴a+b=2．
a＞0，b＞0．
∴

1

a

+

1

b

=

1

2

(a+b)(

1

a

+

1

b

)=

1

2

(2+

b

a

+

a

b

)≥

1

2

(2+2

b a • a b

)=2．当且仅当a=b=1时取等号．
故选B．