sn=[(n+2)/3] an .(1)=>S(n-1)=[(n+1)/3]a(n-1).(2)(1)-(2)Sn-S(n-1)=[(n+2)/3] an -[(n+1)/3]a(n-1)=>an=[(n+2)/3] an -[(n+1)/3]a(n-1)=>[(n+1)/3]a(n-1)={[(n+2)/3] -1}an =[(n-1)/3]an=>an=[(n+1)/(n-1)a(n-1)=>...an=[(n+2)/3] an -[(n+1)/3]a(n-1)这一步到下面的=>[(n+1)/3]a(n-1)={[(n+2)/3] -1}an =[(n-1)/3]an=>an=[(n+1)/(n-1)a(n-1)看不懂Sn-S(n-1)=anan=[(n+2)/3] an -[(n+1)/3]a(n-1)这一步懂，=>[(n+1)/3]a(n-1)={[(n+2)/3] -1}an =[(n-1)/3]an=>an=[(n+1)/(n-1)a(n-1)看不懂移项合并