✔ 最佳答案
先證明 若 d^2 < 4*1999,則
1/(1999-d) + 1/(1999+d) = 2*1999/(1999^2 - d^2) < 2/(1999-4)
proof:
乘開得1999^2 - 4*1999 < 1999^2 - d^2 <=> d^2 < 4*1999 得證
1/1990+1/1991+1/1992+......+1/2008 =
(1/1990 + 1/2008)+(1/1991 + 1/2007) +...+(1/1998 + 1/2000) + 1/1999
< 2/1995 + 2/1995 + ..... + 1/1995 = 19/1995
另外 19/2008 < 1/1990+1/1991+1/1992+......+1/2008
=> 19/2008 < 1/S < 19/1995
=> 105.21..> S > 105
=> [S] = 105
2009-03-05 22:17:08 補充:
SOR! 105.68...>S 才對
打太快想成 1999/19的值