A=1/1.2+1/2.3+1/3.4+...+1/99.100
=> A=1-1/2+1/2-1/3+1/3-1/4+...+1/99-1/100
=> A=1-1/100 (Ta rút gọn từ 1/2->1/99)
=> A=99/100.
Vậy A=99/100.
B=2/3.5+2/5.7+2/7.9+...+2/37.39
=> B=1/3-1/5+1/5-1/7+1/7-1/9+...+1/37-1/39
=> B=1/3-1/39 (Ta rút gọn từ 1/5->1/37)
=> B=13/39-1/39
=> B= 12/39
=> B= 4/13.
Vậy B=4/13.