{nAn}为以a1为首项,d=2的等差数列nAn=1+2(n-1)=2n-1An=(2n-1)/n=2-(1/n)
令 Bn=nAn所以，B(n+1)=Bn+2，B1=1*A1=1易解：Bn=2n-1所以 An=(2n-1)/n
nA=(n-1)A(n-1)+2=~~=A1+2(n-1）=2n-1 An=2-1/n
