A=221​+321​+421​+...+10021​

Ta có : 122=12⋅2<11⋅2221​=2⋅21​<1⋅21​

132=13⋅3<12⋅3321​=3⋅31​<2⋅31​

142=14⋅4<13⋅4421​=4⋅41​<3⋅41​ ...

11002=1100⋅100<199⋅10010021​=100⋅1001​<99⋅1001​

=> A=122+132+142+...+11002<11⋅2+12⋅3+13⋅4+...+199⋅100A=221​+321​+421​+...+10021​<1⋅21​+2⋅31​+3⋅41​+...+99⋅1001​

=> A<11−12+12−13+13−14+...+199−1100A<1

{x + 1.2} x 1221​ + 40%.xx = 4.2

{x+2x+2} x 0,5 + 0,4xx = 8

0,5xx + 1 + 0,4xx = 8

xx.(0,5+ 0,4) = 8 - 1

xx.0,9 = 7

xx = 7 : 0,9

xx = 709970​

Vậy xx = 709970​

Topic 2: Last summer, I had a pleasant experience when I went on a trip to the beach with my family. The weather was perfect, not too hot or too cold. We spent the day swimming in the ocean and playing volleyball on the sand. In the evening, we enjoyed a delicious seafood dinner while watching the sunset. It was a great time to relax and forget about the busy days at work and school. I felt happy to be with my family and enjoy the beauty of nature. It was a simple but very enjoyable day that I will always remember.

1.It's been a few years since I moved to this neighbourhood.

2.While I was waiting for my food, I noticed a nice painting on the wall

1.This is the first time I have made mooncakes for the Mid-Autumn Festival. 

2.I had a desire to explore the hidden gems of the city that most tourists miss

3.Bao wishes he had a big house to get a pet.