原式=1/(x-1)+[1/(x-2)-1/(x-1)]+[1/(x-3)-1/(x-2)]……+[1/(x-100)-1/(x-99)]
=1/(x-1)-1/(x-1)+1/(x-2)-1/(x-2)+……+1/(x-99)-1/(x-99)+1/(x-100)
=0+0+……+1/(x-100)
=1/(x-100)