... du.ng1n(n +1) =1n−1n +1( DS. 1) 7 .1. Gi´o.iha.ncu’a d˜ay sˆo 17 19. an =1 13 +19 12 7+ ···+( 1) n 13 n 1. (DS.34)20. an=2n +1+ 3n +12 n+3n.(DS. 3) 21. an=n +( 1) nn − ( 1) n.(DS. 1) 22. an= 1 n 1 1+ √3 +1 3+√5+ ... r˘a`ng1n(n + 1) (n +2) =12 1n(n +1) 1( n + 1) (n +2)(DS .14 )24. an=1a1a2+1a2a3+ ···+1anan +1. (DS.1a1d)trong d´o {an} l`a cˆa´psˆo´cˆo.ng v´o.i cˆong sai d =0,an=0.25. an= (1 1/ 4) (1 − 1/ 9)··· (1 − 1/ (n ... an=n3n2 +1 3n23n +1. (DS. 1/ 3)7. an=nn +11 −cos n10n.(DS. 1) 8. an=n3+1n2− 1( DS. ∞)9. an=cos n3n−3n6n +1. (DS. 12 )10 . an=( 1) n5√n +1. (DS. 0 )11 . an=√n2 +1+ √n3√n3+ n −√n.(DS. +∞ )12 . an=3 1 − n3+ n.(DS. 0 )13 ....