Question

mn ơi giúp mk với

1.2+2.3+3.4+..+98.99+99.100

Answer 1

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

Answer 2

Đặt A = 1.2+2.3 +.......+99.100

=> 3A = 3.( 1.2+2.3 +.......+99.100 )

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

=> 3A = 0.1.2 + 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ..... + 99.100.101

=> 3A =0.1.2 + 99.100.101

=> 3A = 999900

=> A = 999900 : 3

Vậy A = 333300

Answer 3

Đặt A = 1.2+2.3 +.......+99.100

=> 3A = 3.( 1.2+2.3 +.......+99.100 )

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

=> 3A = 0.1.2 + 1.2.3 - 1.2.3 + 2.3.4 - 2.3.4 + ..... + 99.100.101

=> 3A =0.1.2 + 99.100.101

=> 3A = 999900

=> A = 999900 : 3

Vậy A = 333300

Answer 4

A = 333300