An=3An-1/An-1+3
An=(3A₁)/(3+A₁* (n-1))
Bn=1/An=(3+A₁* (n-1))/(3A₁)
B(n-1)=(3+A₁* (n-2))/(3A₁)
Bn - B(n-1) = A₁/(3A₁) =1/3
数列{Bn}是等差数列