a(n+1)=an/(3an+1)
1/a(n+1) = (3an+1)/an
= 3 + 1/an
1/a(n+1) -1/an =3
=>{ 1/an} 是等差数列 , d=3
1/an -1/a1 = 3(n-1)
1/an -1/1 = 3(n-1)
1/an = 3n-2
an = 1/(3n-2)
