03 hay 0,3 

2: 20=0,3x

=> 2.x =20 .0,3

=>2x =6

=> x=3

1.2+2.3+3.4+4.5+...+99.100

=1.2.3+2.3.3+3.4.3+4.5.3+...+99.100.3

=1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+99.100.(101-98)

=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+99.100.101-98.99.100

=1.2.3-1.2.3+2.3.4-2.3.4+3.4.5-3.4.5+4.5.6-4.5.6+...+99.100.101

=99.100.101=999900

=999900:3=333300

D=1.2+3.4+4.5+...+19.20

3D=1.2.(3-0)+2.3.(4-1)+4.5(6-3)+...+19.20.(21-18)

3D=1.2.3-0.1.2+2.3.4-1.2.3+4.5.6-3.4.5+...+19.20.21-18.19.20

3D=(1.2.3+2.3.4+3.4.5+...+19.20.21)-(0.1.2+1.2.3+2.3.4+...+18.19.20)

3D=19.20.21

3D=7890

D=2660

Gọi A = 1.2 + 2.3 + 3.4 +...+ 99.100

  3A = 1.2.(3 - 0) + 2.3.(4-1) + 3.4.(5-2) +...+ 99.100.(101-98)

    = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +...+ 99.100.101 - 89.99.100

   = 99.100.101 = 999900

A = 999900 : 3 = 333300

vậy A = 333300

Nhân cả 2 vế với 3, ta được: 
3A=1.2.3+ 2.3.3+ 3.4.3+ 4.5.3+...... 99.100.3 
= 1.2.3 + 2.3(4-1) + 3.4.(5-2) +...+ 99.100.(101-98) 
= 1.2.3 + 2.3.4 -1.2.3 + 3.4.5-2.3.4 +...+ 99.100.101-98.99.100 
= 99.100.101 
=> A = (99.100.101):3 
A = 333300 
Vậy A = 333300 

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

A=323400

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 

ban chi can nhan tat co cac do hang voi3 xong sau do ban tinh 

ban chi can nhan cac so hang voi 3 la duoc

đặt là S rồi nhân S và dãy với 3

 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