say

a) tổ tiên

b) quê mùa

x.(x+1)=2+4+6+8+10+...+2500

đẹp hơn chữ mình😢😢

1. Sunrise is a boarding school.
2. It's in Bac Giang
3. Yes
4. They join many interesting clubs, play sports and games. Some students do painting in the art club

1. boarding
2. Sydney
3. mountains and greenfields
4. Dream school
5. English-speaking teachers

zui

Nguồn: Tính tổng: 1x2 + 2x3 + 3x4 +...+ 2019x2020 + 2020x2021 - Hoc24
Đặt $A=1.2+2.3+3.4+.........+2019.2020+2020.2021$

$\Rightarrow 3A=\mathrm{1.2.3}+\mathrm{2.3.3}+\mathrm{3.4.3}+.....+\mathrm{2019.2020.3}+\mathrm{2020.2021.3}$

$=\mathrm{1.2.3}+\mathrm{2.3.}\left(4-1\right)+\mathrm{3.4.}\left(5-2\right)+.....+\mathrm{2020.2021.}\left(2022-2019\right)$

$=\mathrm{1.2.3}+\mathrm{2.3.4}-\mathrm{1.2.3}+\mathrm{3.4.5}-\mathrm{2.3.4}+...+\mathrm{2020.2021.2022}-\mathrm{2019.2020.2021}$

$=\mathrm{2020.2021.2022}$

$\Rightarrow A=\frac{\mathrm{2020.2021.2022}}{3}$

Tính nhanh:
a) 425.7.4-170.60
b) 8.9.14+6.17.12+19.4.18