Đáp án:
Giải thích các bước giải:
= [ 1- 1/2 ] + [ 1- 1/6 ] + [ 1- 1/12 ] + [ 1-1/20 ] + [ 1 - 1/30] + [ 1- 1/42 ] +[ 1- 1/56] +[1-1/72] +[ 1- 90]
= [1+1+1+1+1+1+1+1+1] - [ 1/1.2 +1/ 2.3 + 1/ 3.4+1/4.5+1/5.6+1/6.3+1/7.8+1/8.9+1/9.10]
=9 - [ 1- 1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10]
= 9 - [ 1- 1/10 ]
= 9- 9/10
= 81/10