=1-1/2+1/2-1/3+1/3-1/4+.....+1/n-1/n+1
=1-1/n+1
=n/n+1
Ta có : 1/ 1.2 + 1/ 2.3 + 1/ 3.4 + ... + 1/ n.( n + 1 ) .
= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ..... + 1/n - 1/ n+1 .
= 1 - 1/ n + 1 .
= n+1 / n+1 - 1/ n+1 .
= n/ n+1 .
Đáp sô : n/ n+1
1/1.2 + 1/2.3 + 1/3.4 + ... + 1/n.(n+1)
=1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/n - 1/n+1
=1 - 1/n+1
= n+1/n+1 - 1/ n+1
=n/n+1