\(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+...+\frac{1}{18.19.20}=\frac{1}{2}\left(\frac{2}{1.2.3}+\frac{2}{2.3.4}+...+\frac{2}{18.19.20}\right)\)

\(=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{2.3}+\frac{1}{2.3}-\frac{1}{3.4}+...+\frac{1}{18.19}-\frac{1}{19.20}\right)=\frac{1}{2}\left(\frac{1}{1.2}-\frac{1}{19.20}\right)\)

\(=\frac{1}{2}\left(\frac{1}{2}-\frac{1}{380}\right)=\frac{1}{4}-\frac{1}{760}< \frac{1}{4}\)(ĐPCM)

1.Remember to come to class on time

•-> You must…………  to be on time for class……………………………

2.Remember to do your home work

•-> You must………… remember to do homework……………………………

III,Read about Da Nang City and answer the question :

Da Nang has a population of nearly 800,000 people.The Han River flows through the city.The city part on the east bank is newer and more spacious.The city part on the west bank is more crowed.There are five bridges across the river.The Han River Bridge is the newest one now. The cost of living in Da Nang is the lowest in Central Viet Nam.Da Nang has many beaches.Among them,Non Nuoc beach is one of the most beautiful beaches in the world.But walking in the street on a summer afternoon is not a good idea in Da Nang.There are not many trees so there are not many shadows.It is often very hot at noon .                   

•1. Is Da Nang a big city ? •   

=> Yes,it is

2. Are there many people live in Da Nang ? 

=> Yes,there is

3.Name the famous places in your country or in the world you know.

=> Ha Noi 

=> Tokyo

\(\frac{1}{2^2}+\frac{1}{4^2}+\frac{1}{6^2}+....+\frac{1}{98^2}+\frac{1}{100^2}\)

\(< \frac{1}{1\cdot2}+\frac{1}{2\cdot3}+\frac{1}{3\cdot4}+....+\frac{1}{97\cdot98}+\frac{1}{99\cdot100}\)

\(=\frac{1}{1}-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+....+\frac{1}{99}-\frac{1}{100}\)

\(=1-\frac{1}{100}\)

\(< 1\)