裂项相消：
1／x（x+1）+1／（x+1)(x+2)+1／（x+2）（x+3)+……+1／(x+1998)（x+1999)
=1/x-1/(x+1) + 1/(x+1)-1/(x+2)+……+1/(x+1998)-1/(x+1999)
=1/x-1/(x+1999)
=1998/x(x+1999)