n(n+1)(2n+1)/6 怎么用,比如说求6^2+7^2+8^2+9^2

若(n+1)(n+2)比(n+3)(n+4)大6,求n的值 [3n(n+1)+n(n+1)(2n+1)]/6+n(n+2)化简 [3n(n+1)+n(n+1)(2n+1)]/6+n(n+2)化简 数列a(n)=n (n+1)(n+2)(n+3), 求S(n)怎么用高中数列原理解答? n+(n+1)+(n+2)+(n+3)+(n+4)=5n+10这道题怎么解 lim[n/(n*n+1*1)+n/(n*n+2*2)+...+n/(n*n+n*n)],当x趋向无穷大时,怎么求极限, 2^n/n*(n+1) 证明不等式：(1/n)^n+(2/n)^n+(3/n)^n+.+(n/n)^n 1/[n(n+1)]+1/[(n+1)(n+2)]+1/[(n+2)(n+3)]+1/[(n+3)(n+4)]+[(n+4)(n+5)}怎么算谢谢了, Sn=n(n+2)(n+4)的分项等于1/6[n(n+2)(n+4)(n+5)-(n-1)n(n+2)(n+4)]吗? 求证:3^n>(n+2)2^(n+1)(n>2,n∈N*)用二项式定理 用数学归纳法证明：1*n+2(n-1)+3(n-2)+…+(n-1)*2+n*1=(1/6)n(n+1)(n+2) (n+2)!/(n+1)! 用数学归纳法证明:(n+1)^2+(n+2)^2+……(n+n)^2=n(2n+1)(7n+1)/6就是第Ⅱ步中n=k+1时怎么写 怎么用公式（n+1）*（n+1)-n*n=2n+1解3+5+7+.+2009+2011 Sn=4n+n(n-1)/2乘6=3n^2+n 怎么化简来的? n-2+n-1+n+n+1+n+2怎么算 数列极限的夹逼准则求极限lim[1/n^2+1/(n+1)^2+.+1/(n+n)^2] (n→∞) 设Xn=1/n^2+1/(n+1)^2+.+1/(n+n)^2yn=(n+1)/(n+n)^2≤Xn≤(n+1)/n^2=Zn问：这里yn=(n+1)/(n+n)^2和Zn=(n+1)/n^2是怎么得到的,为什么他们是比Xn小和大的?