  LIM 1 1 1+ + + + + =  − + ÷= 1NVẬY LIM 1

Câu 20. Ta có: 1.2 2.3 3.4¹ + ¹ + ¹ + +^... n n

(

¹ 1

)

.  lim 1 1 1+ + + + + =  − + ÷= 1nVậy ^lim 1.2 2.3 3.4^{1 1 1 ...} n n

(

¹ 1

)

(

¹

)

k +k k+ + + + =1 2 3 ...u =2, ^d=1 nên