Tính nhanh:

Tính nhanh:
a, 3/4 + 2/7 + 1/4 + 3/5 + 5/7
b, 2/17 + 15/23 - 15/17 - 4/19 + 8/23
c, 1/2 x 1/2 + 1/2 x 1/3 + 1/3 x 1/4 + 1/4 x 1/5 + 1/5 x 1/6