1/1*2+1/2*3+1/3*4+...1/2004*2005+1/2005*2006
=1-1/2+1/2-1/3+1/3-1/4+.+1/2004-1/2005+1/2005-1/2006
=1-1/2006
=2005/2006为什么？？？