Đáp án:
Giải thích các bước giải:
A= $\frac{1}{1.3}$+$\frac{1}{3.5}$+....+$\frac{1}{2019.2021}$
=$\frac{1}{1}$-$\frac{1}{3}$+$\frac{1}{3}$-$\frac{1}{5}$+....+ $\frac{1}{2019}$-$\frac{1}{2021}$
=1-$\frac{1}{2021}$
=$\frac{2021}{2021}$-$\frac{1}{2021}$=$\frac{2020}{2021}$
B=$\frac{1}{4.7}$+$\frac{1}{7.10}$+....+$\frac{1}{65.68}$
=$\frac{1}{4}$-$\frac{1}{7}$+$\frac{1}{7}$-$\frac{1}{10}$+....+ $\frac{1}{65}$-$\frac{1}{68}$ =$\frac{1}{4}$-$\frac{1}{68}$
=$\frac{17}{68}$-$\frac{1}{68}$=$\frac{16}{68}$
