Question

[Tạ Huy Hùng]

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

Answer 1

[Sát thủ Killua]

Gọi tổng trên là A

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

A = 1 - 1/2 + 1/2 - 1/3 +.......+ 1/6 - 1/7

A = 1 - 1/7

A = 6/7

Answer 2

[Hoàng Anh Phương]

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

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

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

=1-1/7=7-1/7=6/7