19.(1) 當

時,

······ 2分
又

適合上式
∴

···································································· 3分
由

·························································· 5分
(2)

···································································· 6分
∵

∴

,即

∴ {
cn}的最大項為

··································································· 9分
∴

∴

∴實數
m的取值范圍為

················································ 12分