Question

a) Tính nhanh:

A = 1.2+2.3+3.4+...+2019.2020

b) Áp dụng kết quả phần a) để tính nhanh biểu thức:

B = 1.1+2.2+3.3+...+2019.2019

c) Tính nhanh:

C = 1.2.3+2.3.4+...+48.49.50

Answer 1

A=1.2+2.3+3.4+.............+2019.2020

3A=1.2.3+2.3.3+3.4.3+........................+2019.2020.3

3A=1.2.3+2.3.(4-1)+3.4.(5-2)+..............+2019.2020.(2021-2018)

3A=1.2.3-1.2.3+2.3.4-2.3.4+3.4.5-3.4.5+.............-2018.2019.2020+2019.2020.2021

3A=2019.2020.2021

A=2019.2020.2021 / 3

A=2747468660

Vậy A=2747468660 .

🎀

Answer 2

4c=4.[1.2.3 + 2.3.4 + 3.4.5 + 4.5.6 + . . . . . . + 48.49.50] 
4c=1.2.3.4 +2.3.4.4 +3.4.5.4 +4.5.6.4 +.........+48.49.50.4 
4c=1.2.3.4 +2.3.4.(5-1) + 3.4.5.(6-2) +4.5.6(7-3)+....+ 48.49.50(51-47) 
4c=1.2.3.4 +2.3.4.5 --1.2.3.4 + 3.4.5.6-2.3.4.5 + 4.5.6.7-3.4.5.6+....+ 48.49.50.51-47.48.49.50   =48.49.50.51 
C=(48.49.50.51)/4     = 1499400

Câu b khúc cuối mik ko chc 😥