设an=1+1/2+1/3+.+1/(3n-1)则a(n+1)-an=

问题描述:

设an=1+1/2+1/3+.+1/(3n-1)则a(n+1)-an=

a(n+1) = 1+1/2+1/3+.+1/(3n-1)+...+1/[3(n+1)-1]
= 1+1/2+1/3+.+1/(3n-1)+...+1/(3n+2)
= 1+1/2+1/3+.+1/(3n-1)+1/3n+1/(3n+1)+1/(3n+2)
an = 1+1/2+1/3+.+1/(3n-1)
a(n+1) - an = 1/3n + 1/(3n+1) + 1/(3n+2)