1 . a2 =(3/5)/(6/5+ 1)= 3/ 1 1，

a3=3/ 17，

a4=3/23。

2. Guess an=3/(6n- 1).

The following is proved by mathematical induction:

When n= 1, the formula obviously holds.

Assuming that ak=3/(6k- 1) when n=k, then

a & ltk+ 1 & gt； =[3/(6k- 1)]/[6/(6k- 1)+ 1]= 3/(6k+5)= 3/[6(k+ 1)- 1]，

That is to say, when n=k+ 1, the formula also holds.

For any positive integer n, the formula holds.