an=n*(n+1)*(n+2)分之一 求sn=a1+a2+a3+a4+a5+…………+an为多少?求详解.
问题描述：
an=n*(n+1)*(n+2)分之一 求sn=a1+a2+a3+a4+a5+…………+an为多少?求详解.
答
a‹n›=1/[n(n+1)(n+2)]=(1/2)[1/n(n+1)-1/(n+1)(n+2)]=(1/2){[1/n-1/(n+1)]-[1/(n+1)-1/(n+2)]}故S‹n›=(1/2){[(1-1/2)-(1/2-1/3)]+[(1/2-1/3)-(1/3-1/4)]+[(1/3-1/4)-(1/4-1/5)]+[(1/4-1/5)-...