S = 1.2 + 2.3 + 3.4 + ... + 97.98
=> 3S = 1.2.3 + 2.3.3 + 3.4.3 + ... + 97.98.3
= 1.2.3 + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 97.98.(99 - 96)
= 1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 97.98.99 - 96.97.98
= 97.98.99
= 941 094
=> S = 941 094 : 3 = 313698
Vậy S = 313698