S = 1 * 2 + 2 * 3 + 3 + ... + 99 * 100
3S = 1 * 2 * 3 + 2 * 3 * 4 * ( 4 - 1 ) + ... + 99 * 100 * ( 101 - 98 )
3S = 1 * 2 * 3 + 2* 3 * 4 - 1 * 2 * 3 + ... + 99 * 100 * 101 - 98 * 99 * 100
3S = 99 * 100 * 101 = 999900
S = 999900 : 3 = 333300, nhớ duyệt nha