1/[N(N+2)(N+4)]=1/4{1/[N(N+2)]- 1/[(N+2)(N+4)]}原式=1/4{1/(1*3)-1/(3*5)+1/(3*5)-1/(5*7)+...-1/[N(N+2)]+1/[N(N+2)]- 1/[(N+2)(N+4)]}=1/4{1/3-1/[(N+2)(N+4)]}(有题目知道N是奇数)