Đặt tổng trên = A
Có : 3A = 1.2.3+2.3.3+....+98.99.3
= 1.2.3+2.3.(4-1)+.....+98.99.(100-97)
= 1.2.3+2.3.4-1.2.3+.....+98.99.100-97.98.99
= 98.99.100
=> A = 98.99.100/3 = 323400
k mk nha
Gọi A = 1.2 + 2.3 + .. + 98.99
3A = 1.2.3 + 2.3.3 + ... + 98.99.3
3A = 1.2.3 + 2.3.(4 - 1) + ... + 98.99.(100 - 97)
3A = 1.2.3 + 2.3.4 - 1.2.3 + ... + 98.99.100 - 97.98.99
3A = 98.99.100
3A = 970200
A = 323400
đặt A = 1.2 + 2.3 + 3.4 + ... + 97.98 + 98.99
3A = 1.2.3 + 2.3.3 + 3.4.3 + ... + 97.98.3 + 98.99.3
3A = 1.2.3 + 2.3.(4-1) + 3.4.(5-2) + ... + 97.98.(99-96) + 98.99.(100-97)
3A = 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 97.98.99 - 96.97.98 + 98.99.100 - 97.98.99
3A = 98.99.100
A = 98,99.100 : 3
A = 333300
A=1.2+2.3+3.4+........+98.99
3A=1.2.3+2.3.3+3.4.3+........+98.99.3
3A=1.2.3+2.3.(4 -1) +3.4.(5 -2)+........+98.99.(100 -97)
3A=1.2.3+2.3.4 -1.2.3 +3.4.5 -2.3.4 +........+98.99.100 -97.98.99
3A=98.99.100
===>A=(98.99.100)/3
===A=323400
Vậy A=1.2+2.3+3.4+........+98.99 =323400
Đặt biểu thức là A:
\(A=1.2+2.3+3.4+...+97.97+98.99\)
\(3A=1.2.3+2.3.3+3.4.3+...+98.99.3\)
\(3A=1.2.3+2.3.\left(4-1\right)+3.4.\left(5-2\right)+...+98.99\left(100-97\right)\)
\(3A=1.2.3+2.3.4-1.2.3+2.3.4+...+98.99.100+97.98.98\)
\(A=\frac{\left(98.99.100\right)}{3}=323400\)
