Sn=10n-n^2
Sn-1=10(n-1)-(n-1)^2
an=Sn-Sn-1=10n-n^2-10(n-1)+(n-1)^2=-2n+11
n=1时,a1=10-1=9
-2+11=9,同样满足.
数列{an}的通项公式为an=-2n+11