=1/3*1+1/4*2+1/5*3+1/6*4+……+1/（n+1)(n-1)=1/2*[(1-1/3)+(1/2-1/4)+(1/3-1/5)+(1/4-1/6)+……+1/(n-1)-1/(n+1)]=1/2*(1+1/2-1/n-1/(n+1))=3/4-1/2n-1/2(n+1)
