an=1/(n+1）+2/(n+1)+...+n/(n+1) =(1+2+...+n)/(n+1) =n(n+1)/2(n+1) =n/2∴原式=1/2+2/2+...99/2 =(1+2+...+99)/2 =(1+99)*99/2*2 =25*(100-1) =2475
