Tính tổng A = 4/ 3.7 + 4/ 7.11 + 4/ 11.15 +...+ 4/ 97.111

Tính tổng
a) A = 4/ 3.7 + 4/ 7.11 + 4/ 11.15 +...+ 4/ 97.111
b) B = 2/15 + 2/35 + 2/63 +...+ 2/399
c) C = 3/2^2 . 8/3^2 . 15/4^2 . ... .899/30^2