1 ..    2 = 2 x (1)
2 ..    6 = 2 x (1+2)
3 ..  12 = 2 x (1+2+3)
4 ..  20 = 2 x (1+2+3+4)
5 ..  30 = 2 x (1+2+3+4+5)
.
99 .. S = 2 x (1+2+3+4+ .. +99) = 2 x [⁽⁹⁹⁺¹⁾/₂ x 99] = 2 x 50 x 99 = 9,900