1/2 + 2/3 + 3/4 + 4/5 + 5/6 + 6/7 + 5/6 + 4/5 + 3/4 + 2/3 + 1/2
= (1/2 + 1/2) + (2/3 + 2/3) + (3/4 + 3/4) + (4/5 + 4/5) + (5/6 + 5/6) + 6/7
= 2/2 + 4/3 + 6/4 + 8/5 + 10/6 + 6/7
= 557/70
Vậy 1/2 + 2/3 + 3/4 + 4/5 + 5/6 + 6/7 + 5/6 + 4/5 + 3/4 + 2/3 + 1/2 = 557/70