1x2分之x+2x3分之x+……2005x2006分之x
=x(1-1/2+1/2-1/3+1/3-1/4+...+1/2005-1/2006)
=x(1-1/2006)
=2005x/2006
=2005
所以x=2006
1/n(n+1)=1/n-1/(n+1)