A(n+1)An+3A(n+1)-An=0
1+3/An-1/A(n+1)=0
1/A(n+1)=1+3/An
=3(1/An+1/2)-1/2
1/A(n+1)+1/2=3(1/An+1/2)
此时已证得{1/An+1/2}是等比数列