1/1*2+1/2*3+1/3*4+...+1/n（n+1）==1-1/2+1/2-1/3+...+1/n-1/(n+1)=1-1/(n+1)=n/(n+1)所以是2011/2012
=1-1/2+1/2-1/3+1/3-1/4+。。。。。=1/2012
