Đặt A= 1.2+2.3 +.......+99.100

3A= 1.2.3+2.3.4+3.4.3 +......+ 99.100.3

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

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

3A = 99.100.101 - 0.1.2

3A = 999900 - 0

3A= 999900

A= 999900 : 3

A = 333300 

A=1.2+2.3+3.4+…+99.100

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

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

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

3A = 99.100.101

=> A = \(\frac{99.100.101}{3}\)= 333 300

3A = 1 × 2 × 3 + 2 × 3 × ( 4 - 1 ) + ... + 99 × 100 × ( 101 - 98 )

3A = 1 × 2 × 3 + 2 × 3 × 4 - 1 × 2 × 3 + ... + 99 × 100 × 101 - 98 × 99 × 100

3A = 99 × 100 × 101 = 999900

A = 999900 ÷ 3 = 333300

Tích mình cái nha

           S=1.2+2.3+3.4+4.5+...+98.99+99.100

suy ra :3S=1.2.3+2.3.3+3.4.3+4.5.3+...+98.99.3+99.100.3

            3S=1.2.(3-0)+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+...+98.99.(100-97)+99.100.(101-98)

           3S=1.2.3.0+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+...+98.99.100-97.98.99+99.100.101-98.99.100

           3S=99.100.101

Suy ra :S=99.100.10:3=333300

vậy S=333300

ko bit

=333300

D=1.2+2.3+3.4+...+19.20

=>3D=1.2.3+2.3.3+3.4.3+...+19.20

=1.2.3+2.3(4-1)+3.4(5-2)+...+19.20(21-18)

=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+19.20.21-18.19.20

=19.20.21=7980

=>D=7980:3=2660

Vậy D=2660

Ta có:

3S = 1.2.3 + 2.3.4 + 3.4.3 + ... + 99.100.3

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

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

3S = 99.100.101 - 0.1.2

3S = 999900 - 0

3S = 999900

S = 999900 : 3

S = 333300

Gọi A là biểu thức ta có: 
A = 1.2+2.3+3.4+......+99.100 
Gấp A lên 3 lần ta có: 
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

Thank you very much

333...3x666...6=333...3x(3x222...2)=999...9x222...2=(1000...0-1)x222...2=1000...0x222...2-222...2=222...2000...0-222...22

 A = 1.2 + 2.3 + 3.4 + ... + 2013.2014 
3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + 2013.2014.3 
Mà : 
1.2.3 = 1.2.3 
2.3.3 = 2.3.4 - 2.3.1 
3.4.3 = 3.4.5 - 3.4.2 

2012.2013.3 = 2012.2013.2014 - 2012.2013.2011 
2013.2014.3 = 2013.2014.2015 - 2013.2014.2012 
Cộng tất cả, vế theo vế ---> 3S = 2013.2014.2015 
=> A = 2013.2014.2015 / 3 = 2723058910

tick đã tui mới làm cho

3A=1.2.3+2.3.3+...+n(n+1).3

3A=1.2(3-0)+2.3(4-1)+...+n(n+1)[(n+2)-(n-1)]

3A=(1.2.3-0.1.2)+(2.3.4-1.2.3)+...+[n(n+1)(n+2)-(n-1)n(n+1)]

3A=n(n+1)(n+2)

A=\(\frac{n\left(n+1\right)\left(n+2\right)}{3}\)

=> 3A = 1.2.3 + 2.3.3 + 3.4.3 + .... + 49.50.3

=> 3A = 1.2.3 + 2.3.( 4 - 1 ) + 3.4.( 5 - 2 ) + .... + 49.50.( 51 - 48 )

=> 3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + .... + 49.50.51 - 48.49.50

=> 3A = ( 1.2.3 - 1.2.3 ) + ( 2.3.4 - 2.3.4 ) + .... + ( 48.49.50 - 48.49.50 ) + 49.50.51

=> 3A = 49.50.51

=> A = ( 49.50.51 ) : 3 

=> A = 41650

A = 1.2 + 2.3 + 3.4 + ..... + 49.50

3A=1.2.3+2.3.3+3.4.3+...+49.50.3

3A=1.2.3+2.3.(4-1)+3.4.(5-2)+...+48.49.(50-47)+49.50.(51-48)

3A=1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+48.49.50-47.48.49+49.50.51-48.49.50

3A=(1.2.3-1.2.3)+(2.3.4-2.3.4)+...(47.48.49-47.48.49)-(48.49.50-48.49.50)+49.50.51

3A=0+0+...+0+0+49.50.51

3A=49.50.51

A=\(\frac{49.50.51}{3}\)

A=41650

Đáp số: A=41650

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