1、1*n+2(n-1)+3(n-2)+……+(n-2)*3+(n-1)*2+n*1的和,如何求得1/6n（n+1）（n+2）?2、数列1,-2,2,-3,3,-3,4,-4,4,-4……前100项和是多少 答案怎么是35呢?（北京东城区2008年三模）如图所示，静止在光滑水平面上

证明不等式：(1/n)^n+(2/n)^n+(3/n)^n+.+(n/n)^n 2^n/n*(n+1) [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 化简n分之n-1+n分之n-2+n分之n-3+.+n分之1 化简n分之n-1+n分之n-2+n分之n-3+.+n分之1 化简(n+1)(n+2)(n+3) 当n为正偶数,求证n/(n-1)+n(n-2)/(n-1)(n-3)+...+n(n-2).2/(n-1)(n-3)...1=n (n+2)!/(n+1)! lim2^n +3^n/2^n+1+3^n+1 n(n+1)(n+2)(n+3)+1 因式分解 n(n+1)(n+2)(n+3)+1等于多少 3(n-1)(n+3)-2(n-5)(n-2) lim(n+3)(4-n)/(n-1)(3-2n) lim(n^3+n)/(n^4-3n^2+1) lim[(n+3)/(n+1))]^(n-2) 【n无穷大】 lim(2^n+3^n)^1 (n趋向无穷) 级数n/(n+1)(n+2)(n+3)和是多少