Ta đặt phép tính trên là A

A=12/56+12/140+12/260+....+12/1400

A=6/28+6/70+6/130+...+6/700 3/6

A=3/28+3/70+3/130+....+3/700 3/6.

A=3/4.7+3/7.10+3/10.13+....+3/25.28 3/6

.A=1/4-1/7+1/7-1/10+1/10-1/13+...+1/25-1/28 3/6.

A=(1/4-1/28)+(1/7-1/7)+(1/10+/10)+..+(1/25-1/28) 3/6.

A=7/28-1/28+0+0+...+0 3/6.A=3/14

A=3/14:3/6 A=3/14.6/3 A=3/7

Vậy GTBT=3/7