1. Tính nhanh :
a, A = \frac{3}{54} + \frac{5}{126} + \frac{7}{294} + \frac{8}{609}
b, A = \frac{\frac{3}{54} + \frac{5}{126} + \frac{7}{294}}{\frac{5}{24} + \frac{5}{104} + \frac{5}{234}}
c, \frac{9}{10} + \frac{39}{40} + \frac{87}{88} + ... + \frac{1119}{1120}
2.
a, Cho A = \frac{1}{1.300}...