1/n+2/n+3/n+……+(n-1)/n =[1+2+……+(n-1)]/n =[n(n-1)/2]/n =(n-1)/2 所以原式=1/2+2/2+3/2+……+49/2 =(1+2+……+49)/2 =49*50/2/2 =612.5
