* Tính :
a) A = 1/3 + 1/15 + 1/35 + 1/63 + 1/99
b) B = 7/4 . ( 33/12 + 33/20 + 33/30 + 33/42 + 33/56 )
c) C = 1/1.3 + 1/3.5 + 1/5.7 + ... + 1/2013.2019
d) D = 3^2/1.4 + 3^2/4.7 + 3^2/7.10 + 3^2/10.13 + 3^2/13.16
* Cho abc = 105, bc + b + 1 khác 0. Tính : S = 105/abc + ab + a + b/bc + b + 1 + a/ab + a + 105