Câu trả lời:

1/2+1/6+1/12+1/20+1/30+1/42+1/56

=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+1/6.7+1/7.8

=1/1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8

=1-1/8

=7/8