（1）设等差数列{an}的前n项和为Sn，则Sn＝ n(a1+an) 2 = n(9?2n+11) 2 =-n2+10n；（2）由（1）知，Sn=-n2+10n=-（n-5）2+25，当n=5时Sn有最大值为25．
