Question

Tính tổng sau: A=1.2+2.3+3.4+4.5+5.6+.....+99.100

Answer 1

A = 1.2 + 2.3 + 3.4 + ....... + 99.100

3A = 1.2.3 + 2.3.3 + 3.4.3 + ....... + 99 . 100 . 3

3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2)  +.... + 99.100.(101-98)

3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ..... + 99 . 100 . 101 - 98 . 99 . 100

3A = (1.2.3 - 1.2.3) + (2.3.4-2.3.4) + ... + (98.99.100 - 98.99.100) + 99  . 100 . 101

3A = 99 . 100 . 101 = 999900

A = 999900 : 3 = 333300

Answer 2

A=1*2+2*3+3*4+...+99*100

A=100*101*102:3

A=343400(công thức)

 

 

Answer 3

http://olm.vn/hoi-dap/question/96300.html

Answer 4

 3A = 1.2.3 + 2.3.3 + 3.3.4 + .... + 3.99.100 
Đặt M = 1.2.3 + 2.3.4 + 3.4.5 + .... + 99.100.101 
=> M - 3A = 1.2.3 - 1.2.3 + 2.3.(4-3) + 3.4 ( 5-3) + .... + 99.100 ( 101 -3) 
= 1.2.3 + 2.3.4 + .... + 98.99.100 
=> M -3A = M - 99.100.101 
=> A = 99.100.101/3 = 333300

Answer 5

 3A = 1.2.3 + 2.3.3 + 3.3.4 + .... + 3.99.100 
Đặt M = 1.2.3 + 2.3.4 + 3.4.5 + .... + 99.100.101 
=> M - 3A = 1.2.3 - 1.2.3 + 2.3.(4-3) + 3.4 ( 5-3) + .... + 99.100 ( 101 -3) 
= 1.2.3 + 2.3.4 + .... + 98.99.100 
=> M -3A = M - 99.100.101 
=> A = 99.100.101/3 = 333300  

Answer 6

Thanks các bạn nhìu

Answer 7

bài thi lớp 6 thì phải