有数学归纳法证明:1+1/(1+2)+1/(1+2+3))+…+1/(1+2+3+…+n)=n^2/(n+1)时,由n-k到n=k+1左边需要添加的项是______________.
有数学归纳法证明:1+1/(1+2)+1/(1+2+3)+…+1/(1+2+3+…+n)=n^2/(n+1)时,由n=k到n=k+1左边需要添加的项是______________.