=>3D =1.2.3 + 2.3.3 + 3.4.3 + ..... + 99.100 .3
=> 3D = 1.2.3 - 2.3. ( 4-1) + 3.4. (5-2) + ... + 98.99 (100 - 97 ) + 99 . 100 . ( 101-98)
=> 3D= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +... + 98.99.100 -97.98.99 +99.100.101-98.99.100
=> 3D= 99.100.101
=> 3D= 999 900
D= 999 900 .3 = 333 300
=>3D =1.2.3 + 2.3.3 + 3.4.3 + ..... + 99.100 .3
=> 3D = 1.2.3 - 2.3. ( 4-1) + 3.4. (5-2) + ... + 98.99 (100 - 97 ) + 99 . 100 . ( 101-98)
=> 3D= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +... + 98.99.100 -97.98.99 +99.100.101-98.99.100
=> 3D= 99.100.101
=> 3D= 999 900
D= 999 900 .3 = 333 300
Đặt D = 1.2 + 2.3 + 3.4 + ........ + 99.100
=> 3D = 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ...... + 99.100.101
=> 3D = 99.100.101
=> D = 99.100.101/3
=> D = 333300
=>3D =1.2.3 + 2.3.3 + 3.4.3 + ..... + 99.100 .3
=> 3D = 1.2.3 - 2.3. ( 4-1) + 3.4. (5-2) + ... + 98.99 (100 - 97 ) + 99 . 100 . ( 101-98)
=> 3D= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +... + 98.99.100 -97.98.99 +99.100.101-98.99.100
=> 3D= 99.100.101
=> 3D= 999 900
Đặt A = 1x2 + 2x3 + 3x4 + 4x5 +.............+ 99x100
Ta có :
=>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) + ... + 98.99 (100 - 97 ) + 99 . 100 . ( 101-98)
=> 3A= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +... + 98.99.100 -97.98.99 +99.100.101-98.99.100
=> 3A= 99.100.101
=> 3A= 999 900
A= 999 900 .3
A = 333 300
=> 3D =1.2.3 + 2.3.3 + 3.4.3 + ..... + 99.100 .3
=> 3D = 1.2.3 - 2.3. ( 4-1) + 3.4. (5-2) + ... + 98.99 (100 - 97 ) + 99 . 100 . ( 101-98)
=> 3D= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 +... + 98.99.100 -97.98.99 +99.100.101-98.99.100
=> 3D= 99.100.101
=> 3D= 999 900
S = 1x2 + 2x3 + 3x4 + 4x5 + ...+ 99x100
S x 3 = 1x2x3 + 2x3x3 + 3x4x3 + 4x5x3 + ... + 99x100x3
S x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + 4x5x(6-3) + ... + 99x100x(101-98)
S x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + 4x5x6 - 3x4x5 + ... + 99x100x101 - 98x99x100.
S x 3 = 99x100x101
S = 99x100x101 : 3
S = 333300
A = 1x2 + 2x3 + 3x4 + 4x5 + ...+ 99x100
A x 3 = 1x2x3 + 2x3x3 + 3x4x3 + 4x5x3 + ... + 99x100x3
A x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + 4x5x(6-3) + ... + 99x100x(101-98)
A x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + 4x5x6 - 3x4x5 + ... + 99x100x101 - 98x99x100.
A x 3 = 99x100x101
A = 99x100x101 : 3
A = 333300
Gọi biểu thức trên là A, ta có :
A = 1x2 + 2x3 + 3x4 + 4x5 + ...+ 99x100
A x 3 = 1x2x3 + 2x3x3 + 3x4x3 + 4x5x3 + ... + 99x100x3
A x 3 = 1x2x3 + 2x3x(4-1) + 3x4x(5-2) + 4x5x(6-3) + ... + 99x100x(101-98)
A x 3 = 1x2x3 + 2x3x4 - 1x2x3 + 3x4x5 - 2x3x4 + 4x5x6 - 3x4x5 + ... + 99x100x101 - 98x99x100.
A x 3 = 99x100x101
A = 99x100x101 : 3
A = 333300
