Tính: 1/2.3 + 1/3.4 + 1/4.5 + 1/5

a) 1/2.3 + 1/3.4 + 1/4.5 + 1/5.6
b) 1/5.6 + 1/6.7 + 1/7.8 + 1/8.9 + 1/9.10
c) 1/2 + 1/6 + 1/12 + ...... + 1/72 + 1/90
d) 2/3.5 + 2/5.7 + 2/7.9 + 2/9.10