`S = 1.2 + 2.3 + 3.4 + 4.5 + ... + 99.100.`

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

`3S =  1.2.3 + 2.3.4-1.2.3 + 3.4.5-4.5.6 + 4.5.6-3.4.5 + ... + 99.100.101-98.99.100`

`3S =  99.100.101`

`S = 33.100.101`

`S = 333300`

3S=1.2(3-0)+2.3(4-1)+.....+99.100(101-98)

=1.2.3-0.1.2+2.3.4-1.2.3+4.5.6-2.3.4+....+99.100.101-98-99-100

=99.100.101

S=33.100.101

=333300

Gọi tổng là A

3.A=1.2.3+2.3.3+3.4.3+...+99.100.3

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

=(1.2.3-0.1.2)+(2.3.4-1.2.3)+(3.4.5-2.3.4)+...+(99.100.101-98.99.100)

=99.100.101-0.1.2(vì những số khác giản ước)

=999900-0

=999900

A=999900:3=333300

Vậy A=333300

Đặt P = 1.2+2.3+3.4+...+99.100

3P = 1.2.3+2.3.3+3.4.3+...+99.100+3

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

3P = ( 1.2.3 + 2.3.4 + 3.4.5 + 99.100.101 ) -( 0.1.2 + 1.2.3 + 2.3.4 + ....+ 98.99.100)

3P = 99.100.101 - 0.1.2

3P = 999900 - 0

3P = 999900

P = 999900 : 3

P = 333300

số số hạng là : ( 99.100 - 1.2 ) / 1.1 +1=90 ( số )

tổng của S là : ( 1.2 + 99.100 ) * 90 / 2=4513.5

tìm số số hạng rồi tìm tổng

S = 1.2 + 2.3 + 3.4 + ... + 99.100

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

3S = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 99.100.101 - 98.99.100

3S = 99.100.101

S = 33.100.101

S = 333 300

Ủng hộ mk nha ^_-

Ta có: S = 1.2 + 2.3 + 3.4 + ... + 99.100

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

=> 3S = 1.2.3 - 1.2.3 + 2.3.4 -  2.3.4 + ... - 99.100.101 + 98.99.100

=> 3S = 98.99.100

=> S = \(\frac{98.99.100}{3}=333300\)

A . 3 = 1.2.3 + 2.3.3 + 3.4.3 + … + 99.100.3 
A . 3 = 1.2.3 + 2.3.(4 - 1) + 3.4.( 5 - 2) + … + 99.100. (101 - 98) 
A . 3 = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + … + 99.100.101 - 98.99.100 
A . 3 = 99.100.101 
A = 99.100.101 : 3 
A = 33.100.101 
A = 333 300

S = 1.2 + 2.3 + 3.4 + ..... + 99. 100

=> 3.S = 1.2.3+2.3.3+3.4.3+......+99.100.3

=> 3.S = 1.2.( 3 - 0) + 2 . 3 . ( 4 - 1 ) + 3. 4 . ( 5 - 2 ) + ..... + 99 . 100 . ( 101 - 98 )

=> 3 . S = ( 1.2 . 3 - 1 . 2 . 0 ) + ( 2 . 3 . 4 - 2 . 3 . 1 ) + ...... + ( 99. 100 . 101 - 98 . 99 . 100 )

=> 3.S = 99 . 100 . 101 - 1 . 2 .0

=> 3.S = 999 900 - 0

=> 3 . S = 999 900

=> S = 333 300

Vậy: S = 333 300

S = 1.2 + 2.3 + 3.4 + ....+  99.100

3S = 1.2.3 + 2.3.(4-1) +....+ 99.100.(101-98)

3S = 1.2.3 + 2.3.4 - 1.2.3 + ..... + 99.100.101-98.99.100

3S=  99.100.101 = 999900

S = 999900 : 3 = 333300

S = 1.2 + 2.3 + 3.4 +...+ 99.100

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

3S = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +...+ 99.100.101 - 98.99.100

3S = 99.100.101

3S = 999900

S = 333300

D=1.2+2.3+3.4+...+99.100

=>3D=1.2.3+2.3.3+3.4.3+...+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=999900

=>D=333300

Đặt S= 1.2 + 2.3 + 3.4 + ...+ 99.100
 3S = 1.2.3+2.3.3+3.4.3+...+98.99.3+99.100.3
3S= 1.2.3+2.3(4-1)+3.4(5-2)+...+98.99(100-97)+99.100(101-98)
3S= 1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...-97.98.99+99.100.101-98.99.100
3S = 99.100.101  3S = 3.33.100.101 
 S=33.100.101= 333300

S = (99x100x101 - 0x1x2):3 = 333300

các bạn cho mình vài li-ke cho tròn 550 với 

Vuthithu 2002