咸阳怀古翻译成现代文:(n+2)^2-n^2=4*(n+1)

n/(n-2)+n/(n-3)+n/(n-4)+n/(n-5)+...+2/(-1)=? "1^n+2^n+3^n......+m^n=? 1^n+2^n+3^n......+m^n= x=n*(n+1)*(n+2)*(n+3)....... (n+2)^2-n^2=4*(n+1) 求和：1*2+2*3+3*4+…+(n-1)*n+n*(n+1)=? lim（n→∞）[1/（n^2+n+1）+2/(n^2+n+2)+.....+n/(n^2+n+n)]=? 证明C（0，n)^2+C(1,n)^2+……+C(n,n)^2=C(n,2n) 1*1!+2*2!+~~~~~~~~~+n*n!=? 1×1+2×2+......+n×n=?