举报
假_老_练
只是说这儿容易混淆:3Sn-1 VS 3S(n-1)。 3Sn^2=an(3Sn-1) =(Sn-S(n-1))(3Sn-1) =3Sn^2-3SnS(n-1)-Sn+S(n-1) =3Sn^2-3SnS(n-1)-an --> an=-3SnS(n-1) =-3S(n-1)^2-3anS(n-1)) --> an=-3S(n-1)^2/(3S(n-1)+1) Sn=S(n-1)+an =S(n-1)-3S(n-1)^2/(3S(n-1)+1) =(3S(n-1)^2+S(n-1)-3S(n-1)^2)/(3S(n-1)+1) =S(n-1)/(3S(n-1)+1) --> 1/Sn=3+1/S(n-1) =3+3+1/S(n-2) =... =3*(n-1)+1/S1 =3n-2 -->Sn=1/(3n-2) -->an=-3SnS(n-1)=-3/(3n-2)(3n-5)