{a1+8/7*a2+(8/7)²*a3+...(8/7)^(n-1)an}-{a1+8/7*a2+(8/7)²*a3+...(8/7)^(n-2)a(n-1)}=n(n+1)-(n-1)n=2n
即有(8/7)^(n-1)an=2n
an=2n(7/8)^(n-1)
{a1+8/7*a2+(8/7)²*a3+...(8/7)^(n-1)an}-{a1+8/7*a2+(8/7)²*a3+...(8/7)^(n-2)a(n-1)}=n(n+1)-(n-1)n=2n
即有(8/7)^(n-1)an=2n
an=2n(7/8)^(n-1)