Sn=[1+1/a+1/a²+...+1/a^(n-1)]+[1+4+...+(3n-2)]=[1+1/a+1/a²+...+1/a^(n-1)]+3(1+2+...+n)-2n=[1+1/a+1/a²+1/a^(n-1)]+3n(n+1)/2 -2n=[1+1/a+1/a²+1/a^(n-1)]+ n(3n-1)/2a=1时,1+1/a+1/a²...