Question

1.2+2.3+3.4+.....+98.99

các bạn giúp tớ nhé

ai nhanh mk tick

 

Answer 1

A=1.2+2.3+3.4+........+98.99 
3A=1.2.3+2.3.3+3.4.3+........+98.99.3 
3A=1.2.3+2.3.(4 -1) +3.4.(5 -2)+........+98.99.(100 -97) 
3A=1.2.3+2.3.4 -1.2.3 +3.4.5 -2.3.4 +........+98.99.100 -97.98.99 
3A=98.99.100 
===>A=(98.99.100)/3

#Japhkiel#

Answer 2

A=1.2+2.3+3.4+........+98.99 
3A=1.2.3+2.3.3+3.4.3+........+98.99.3 
3A=1.2.3+2.3.(4 -1) +3.4.(5 -2)+........+98.99.(100 -97) 
3A=1.2.3+2.3.4 -1.2.3 +3.4.5 -2.3.4 +........+98.99.100 -97.98.99 
3A=98.99.100 
A=\(\frac{98.99.100}{3}=\frac{970200}{3}=323400\)

Answer 3

Đặt A\(=1\cdot2+2\cdot3+3\cdot4+......+98\cdot99\), ta có

\(3A=1\cdot2\cdot3+2\cdot3\cdot3+.....+98\cdot99\cdot3\)

\(=1\cdot2\cdot3+2\cdot3\cdot\left(4-1\right)+....+98\cdot99\cdot\left(100-97\right)\)

\(=1\cdot2\cdot3+2\cdot3\cdot4-1\cdot2\cdot3+....+98\cdot99\cdot100-97\cdot98\cdot99\)

\(=98\cdot99\cdot100\)

\(\Rightarrow A=\frac{98\cdot99\cdot100}{3}=323400\)

Answer 4

cÁI BÀ HIỀN NÀY VÀO ĐÂY CHẮC ĐỂ CHÉP BÀI