19.(1) 当
时,
······ 2分
又
适合上式
∴
···································································· 3分
由
·························································· 5分
(2)
···································································· 6分
∵
∴
,即
∴ {
cn}的最大项为
··································································· 9分
∴
∴
∴实数
m的取值范围为
················································ 12分