n*(n+1)分之1 = n分之1 - （n+1）分之1
原式 = 1 - 1/2 + 1/2 - 1/3 + .+ 1/2004 - 1/2005 + 1/2005 - 1/2006
= 1 - 1/2006 = 2005/2006