（2）?a1?a3???a25?130a2?a4???a26?143?13k
?两式相减：13d?13?13k
13(13?1)?13a1??2d?130
2?d?1?k
?a3??2?12k
?an?a1?(n?1)d?(1?kn?(13k?3))
22?cn?an?an?1?(an?an?1)(an?an?1)
?26k2?32?6?(2n?1)(1?k2)??2(1?k)2?n?25k?30k?5
（3）因为当且仅当n?12时Sn最大 ?有c12?0,c13?0
22????24(1?k)?25k?30k?5?0?k?18k?19?0??2即? 22???36(1?k)?25k?30k?5?0??k?22k?21?0
?k?1或k??19???k??19或k?21k?21或k?1?