<1绝对收敛 当li (-b) k+1 k→0a 1发散 < lim 绝对收敛 → 故当(2-b k 发散 故R=2=lim k+1( ) ( ) ( ) î í ì > < = × - - - + ® ¥ + + ® ¥ 发散 绝对收敛 当 1 1 lim lim 1 1 1 z b a a a z b a z b k k k k k k k k ( ) ï ï î ï ï í ì > < - + ® ¥ + ® ¥ 发散 绝对收敛 故当 1 1 lim lim k k k k k k a a a a z b 1 lim 1 + ® ¥ = = k k k a a l 故 R