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

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

S = 33.100.101 = 333300

Vậy S bằng 333300

Đáp số : S : 333300

Đặt S=1.2.3+2.3.4+....+99.100.101

3S=1.2.3+2.3.3+3.4.3+....+98.99.3+100.101.3

3S=1.2.3+2.3(4-1)+3.4(5-2)+.....+98.99+99.100(101-98)

3S=1.2.3+2.3.4-1.2.3+3.4.5+3.4.5-2.3.4+......+97.98.99+99.100.101-98.99.100

3S=33.100.101=333300

Vậy S=333300

Đặt \(A=1.2.3+2.3.4+3.4.5+...+99.100.101\)

\(\Rightarrow4A=1.2.3.4+2.3.4.4+...+99.100.101.4\)

\(=1.2.3\left(4-0\right)+2.3.4\left(5-1\right)+...+99.100.101\left(102-98\right)\)

\(=\left(1.2.3.4+2.3.4.5+...+99.100+101.102\right)-\left(0.1.2.3+1.2.3.4+...+98.99.100.101\right)\)

\(=99.100.101.102-0.1.2.3\)

\(=101989800\)

\(\Rightarrow A=101989800:4=25497450\)

Vậy \(A=25497450.\)

Đặt A = 1.2.3 + 2.3.4 + ... + 99.100.101

=> 4A = 1.2.3.4 + 2.3.4.(5-1) + ... + 99.100.101.(102-98)

=> 4A = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + ... + 99.100.101.102 - 98.99.100.101

=> 4A = 99.100.101.102

=> 4A = 101989800

=> A = 25497450

Nhân 4 p lên rồi trừ đi p còn 3 p là xong

Dễ mà ~ Suy nghĩ đi ~~~

1.2.3 = 1/4 . (1.2.3.4 - 0.1.2.3)

2.3.4 = 1/4 . (2.3.4.5 - 1.2.3.4)

3.4.5 = 1/4 . (3.4.5.6 - 2.3.4.5)

.................

99.100.101 = 1/4 . (99.100.101.102 - 98.99.100.101)

C = 1.2.3+2.3.4+3.4.5+.........+99.100.101

C= 1/4 . (99.100.101.102 - 98.99.100.101)

CHUC BN HOK GIỎI!

25497450

=> 4E = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + .... + 99.100.101.4

=> 4E = 1.2.3.( 4 - 0 ) + 2.3.4.( 5 - 1 ) + 3.4.5.( 6 - 2 ) + .... + 99.100.101.( 102 - 98 )

=> 4E = 1.2.3.4 - 0.1.2.3 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + ... + 99.100.101.102 - 98.99.100.101

=> 4E = ( 1.2.3.4 - 1.2.3.4 ) + ( 2.3.4.5 - 2.3.4.5 ) +  + ... + ( 98.99.100.101 - 98.99.100.101 ) + 99.100.101.102

=> 4E = 99.100.101.102

=> E = ( 99.100.101.102 ) : 4

Đặt A=1/1.2.3+1/2.3.4+...+1/99.100.101 

2A=2/1.2.3+2/2.3.4+...2/99.100.101

2A=3-1/1.2.3+4-2/2.3.4+...+101-99/99.100.101

2A=3/1.2.3-1/1.2.3+4/2.3.4-2/2.3.4+...+101/99.100.101-99/99.100.101

2A=1/1.2-1/2.3+1/2.3-1/3.4+...+1/99.100-1/100.101

2A=1/2-1/10100

\(A=1.2.3+2.3.4+...+99.100.101\)

\(\Rightarrow4A=1.2.3.4+2.3.4\left(5-1\right)+...+99.100.101.\left(102-98\right)\)

\(=1.2.3.4+2.3.4.5-1.2.3.4+...+99.100.101.102-98.99.100.101\)

\(=99.100.101.102\)

\(\Rightarrow A=\dfrac{99.100.101.102}{4}=99.25.101.102\)

Vậy...